Review deprecations for potential removal

This is a placeholder issue for Traits 7.0: a reminder to review the various pieces of deprecated functionality in the Traits codebase and consider removing them.

We'll need to make the decision on a case-by-case basis - some of the deprecated features have little evidence of wider usage and are fairly safe to remove; some may need to be treated more carefully, with removal possibly deferred until Traits 8.0 or later (for example the `Unicode` trait).

Note: `on_trait_change` is not currently deprecated, and will _not_ be removed in Traits 7.0!

Here's a preliminary list of candidates for removal:

- [ ] The ability to call `CTrait.default_value` with a nonzero number of arguments.
- [ ] `configure_traits` support for restoration from a pickle file
- [ ] The ability of `provides` to perform interface checking when `CHECK_INTERFACES` is nonzero
- [ ] The `typecode` parameter to `Array` and `CArray`
- [ ] Support for `rich_compare` metadata on traits
- [ ] The `traits.testing.nose_tools` package
- [ ] `_TraitsChangeCollector` support for the `trait` parameter
- [ ] `HasTraits.set` and `HasTraits.get`
- [ ] `SingletonHasTraits`, `SingletonHasStrictTraits` and `SingletonHasPrivateTraits`
- [ ] `MetaInterface` implicit adaptation on instantiation
- [ ] `TraitPrefixList`, `TraitMap`, `TraitPrefixMap`
- [ ] `TraitTuple`, `TraitList` and `TraitDict`
- [ ] `Color`, `RGBColor`, `Font`
- [ ] The `Long` trait type
- [ ] The `Unicode` trait type (this one will likely have to live beyond Traits 7.0)
- [ ] `AdaptedTo` as an alias for `Supports`
- [ ] `false`, `true`, `undefined` trait type aliases
- [ ] `ListInt`, `ListThis` and other `List*` and `Dict*` trait types