[libc++] Remove macros for keeping std::allocator members and void...

[libc++] Remove macros for keeping std::allocator members and void specialization after C++20 (#85806)

Fixes #75975.

Remove `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS` for the LLVM 19
release, it was previously marked as deprecated in LLVM 18.

I believe that
`_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION` was only
used by Google in conjunction with
`_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`.

Removing both macros together should not cause any issues in practice,
even though we did not announce the removal of
`_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION` before.