#98 breaks assignment in Boost.Fusion

The fix in #98 breaks usage in `boost::fusion::deque`.
Commenting out the `#if[...]#endif` around the operator definition make the code below compile again.

Sadly I encountered this when using Boost.Spirit, so the cvise-reduced case is still quite icky.
Unsure if this is a bug in Boost.Variant, or in how Boost.Fusion uses it.

Godbolt: https://godbolt.org/z/s8EfxT8r4

```
In file included from :2:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3.hpp:62:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/auxiliary.hpp:11:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/auxiliary/any_parser.hpp:17:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/move_to.hpp:12:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/attribute_category.hpp:16:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/support/traits/is_variant.hpp:11:
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/variant.hpp:17:
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:620:13: error: object of type 'boost::fusion::deque, bool>' cannot be assigned because its copy assignment operator is implicitly deleted
620 | lhs = rhs_;
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:1018:16: note: in instantiation of member function 'boost::detail::variant::direct_assigner, bool>>::operator()' requested here
1018 | return visitor_(operand);
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:112:20: note: in instantiation of function template specialization 'boost::detail::variant::invoke_visitor, bool>>, false>::internal_visit, bool> &>' requested here
112 | return visitor.internal_visit(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:152:13: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl_invoke_impl, bool>>, false>, void *, boost::fusion::deque, bool>>' requested here
152 | return (visitation_impl_invoke_impl)(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/detail/visitation_impl.hpp:235:11: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl_invoke, bool>>, false>, void *, boost::fusion::deque, bool>, boost::variant, bool>>::has_fallback_type_>' requested here
235 | , BOOST_VARIANT_AUX_APPLY_VISITOR_STEP_CASE
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/repetition/repeat.hpp:30:26: note: expanded from macro 'BOOST_PP_REPEAT'
30 | # define BOOST_PP_REPEAT BOOST_PP_CAT(BOOST_PP_REPEAT_, BOOST_PP_AUTO_REC(BOOST_PP_REPEAT_P, 4))
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/cat.hpp:22:32: note: expanded from macro 'BOOST_PP_CAT'
22 | # define BOOST_PP_CAT(a, b) BOOST_PP_CAT_I(a, b)
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/preprocessor/cat.hpp:29:34: note: expanded from macro 'BOOST_PP_CAT_I'
29 | # define BOOST_PP_CAT_I(a, b) a ## b
| ^
:148:1: note: expanded from here
148 | BOOST_PP_REPEAT_1
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:2282:33: note: in instantiation of function template specialization 'boost::detail::variant::visitation_impl, boost::detail::variant::visitation_impl_step, char, boost::mpl::l_item, boost::fusion::deque, bool>, boost::mpl::l_end>>>, boost::mpl::l_iter>, boost::detail::variant::invoke_visitor, bool>>, false>, void *, boost::variant, bool>>::has_fallback_type_>' requested here
2282 | return detail::variant::visitation_impl(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/variant/variant.hpp:2294:16: note: (skipping 12 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
2294 | return internal_apply_visitor_impl(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:209:26: note: in instantiation of function template specialization 'boost::spirit::x3::action, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>::parse<__gnu_cxx::__normal_iterator>, boost::spirit::x3::context, const boost::spirit::x3::action, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>>, boost::spirit::x3::unused_type>' requested here
209 | bool r = rhs.parse(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:265:20: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser, boost::spirit::x3::rule>::parse_rhs_main, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>, __gnu_cxx::__normal_iterator>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, const boost::spirit::x3::unused_type>' requested here
265 | return parse_rhs_main(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:290:20: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser, boost::spirit::x3::rule>::parse_rhs_main, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>, __gnu_cxx::__normal_iterator>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, const boost::spirit::x3::unused_type>' requested here
290 | return parse_rhs_main(rhs, first, last, context, rcontext, unused);
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/spirit/home/x3/nonterminal/detail/rule.hpp:328:28: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser, boost::spirit::x3::rule>::parse_rhs, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>, __gnu_cxx::__normal_iterator>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type>' requested here
328 | ok_parse = parse_rhs(rhs, first, last, context, attr_, attr_
| ^
:19:45: note: in instantiation of function template specialization 'boost::spirit::x3::detail::rule_parser, boost::spirit::x3::rule>::call_rule_definition, boost::spirit::x3::sequence>, boost::spirit::x3::matches_directive>>>, (lambda at :9:25)>, __gnu_cxx::__normal_iterator>, boost::spirit::x3::unused_type, boost::spirit::x3::unused_type, mpl_::bool_>' requested here
19 | x3::detail ::rule_parser::call_rule_definition(
| ^
/opt/compiler-explorer/libs/boost_1_84_0/boost/fusion/container/deque/deque.hpp:121:9: note: copy assignment operator is implicitly deleted because 'deque, bool>' has a user-declared move constructor
121 | deque(deque&& seq)
| ^
```