[libc++] Remove the _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS escape hatch

[ldionne]

As described in #69994, using the escape hatch makes us non-conforming in C++20. There are other problems like #63900.

We discussed this in the libc++ monthly meeting today and agreed that we should deprecate the macro in LLVM 18 and remove it in LLVM 19.

[ldionne]

See https://clang.llvm.org/docs/LanguageExtensions.html#deprecating-macros