[libc++] Removes _LIBCPP_AVAILABILITY_TO_CHARS.

After moving the std::to_chars base 10 implementation from the dylib to
the header the integral overloads of std::to_chars are available on all
platforms.

Remove the _LIBCPP_AVAILABILITY_TO_CHARS availability macro and update
the tests.

Depends on D125704

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D125745