Remove Boost.StaticAssert submodule

Since Boost.StaticAssert has been [merged](https://github.com/boostorg/config/pull/531) into Boost.Config, this submodule can be removed once all Boost libraries have removed references to it. For tracking purposes, here is the list of the relevant PRs:

- https://github.com/boostorg/accumulators/pull/67
- https://github.com/boostorg/algorithm/pull/130
- https://github.com/boostorg/align/pull/24
- https://github.com/boostorg/assert/pull/44
- https://github.com/boostorg/assign/pull/50
- https://github.com/boostorg/array/pull/30
- https://github.com/boostorg/beast/pull/3074
- https://github.com/boostorg/bimap/pull/51
- https://github.com/boostorg/callable_traits/pull/201
- https://github.com/boostorg/charconv/pull/287
- https://github.com/boostorg/chrono/pull/81
- https://github.com/boostorg/circular_buffer/pull/54
- https://github.com/boostorg/cobalt/pull/262
- https://github.com/boostorg/compute/pull/898
- https://github.com/boostorg/concept_check/pull/47
- https://github.com/boostorg/config/pull/533
- https://github.com/boostorg/contract/pull/41
- https://github.com/boostorg/date_time/pull/250
- https://github.com/boostorg/endian/pull/60
- https://github.com/boostorg/function/pull/57
- https://github.com/boostorg/function_types/pull/19
- https://github.com/boostorg/fusion/pull/279
- https://github.com/boostorg/geometry/pull/1447
- https://github.com/boostorg/gil/pull/786
- https://github.com/boostorg/graph/pull/450
- https://github.com/boostorg/graph_parallel/pull/45
- https://github.com/boostorg/hash2/pull/46
- https://github.com/boostorg/heap/pull/57
- https://github.com/boostorg/histogram/pull/419
- https://github.com/boostorg/icl/pull/52
- https://github.com/boostorg/interval/pull/48
- https://github.com/boostorg/io/pull/17
- https://github.com/boostorg/iostreams/pull/190
- https://github.com/boostorg/lexical_cast/pull/95
- https://github.com/boostorg/lockfree/pull/126
- https://github.com/boostorg/math/pull/1351
- https://github.com/boostorg/metaparse/pull/22
- https://github.com/boostorg/move/pull/63
- https://github.com/boostorg/mp11/pull/112
- https://github.com/boostorg/mpi/pull/177
- https://github.com/boostorg/mpl/pull/91
- https://github.com/boostorg/mqtt5/pull/46
- https://github.com/boostorg/multi_array/pull/42
- https://github.com/boostorg/multi_index/pull/91
- https://github.com/boostorg/mysql/pull/497
- https://github.com/boostorg/odeint/pull/105
- https://github.com/boostorg/pfr/pull/229
- https://github.com/boostorg/phoenix/pull/125
- https://github.com/boostorg/program_options/pull/155
- https://github.com/boostorg/property_map/pull/36
- https://github.com/boostorg/property_map_parallel/pull/4
- https://github.com/boostorg/property_tree/pull/134
- https://github.com/boostorg/proto/pull/39
- https://github.com/boostorg/ptr_container/pull/44
- https://github.com/boostorg/python/pull/509
- https://github.com/boostorg/random/pull/152
- https://github.com/boostorg/range/pull/162
- https://github.com/boostorg/rational/pull/57
- https://github.com/boostorg/redis/pull/386
- https://github.com/boostorg/regex/pull/269
- https://github.com/boostorg/serialization/pull/340
- https://github.com/boostorg/signals2/pull/84
- https://github.com/boostorg/smart_ptr/pull/122
- https://github.com/boostorg/sort/pull/109
- https://github.com/boostorg/spirit/pull/846
- https://github.com/boostorg/stacktrace/pull/223
- https://github.com/boostorg/statechart/pull/27
- https://github.com/boostorg/system/pull/139
- https://github.com/boostorg/test/pull/474
- https://github.com/boostorg/thread/pull/417
- https://github.com/boostorg/tuple/pull/32
- https://github.com/boostorg/type_traits/pull/203
- https://github.com/boostorg/typeof/pull/27
- https://github.com/boostorg/ublas/pull/198
- https://github.com/boostorg/units/pull/66
- https://github.com/boostorg/unordered/pull/341
- https://github.com/boostorg/uuid/pull/192
- https://github.com/boostorg/variant/pull/115
- https://github.com/boostorg/wave/pull/245
- https://github.com/boostorg/xpressive/pull/29

- https://github.com/boostorg/boostdep/pull/33
- https://github.com/boostorg/cmake/pull/103
- https://github.com/boostorg/boost-ci/pull/313

Once these PRs are merged and propagate to master, I propose to add a release note for the next Boost release to notify users that the submodule is deprecated and will soon be removed. Then, after 3 Boost releases, actually remove the submodule.

CC @pdimov