[libc++] newlib/xlocale.h: remove redundant includes

This permits using the <locale> header with newlib or picolibc when
LIBCXX_ENABLE_WIDE_CHARACTERS=FALSE.

Since D136682 the __support/xlocale headers themselves include the
headers they require, respecting the LIBCXX_ENABLE_WIDE_CHARACTERS
option.

Differential Revision: https://reviews.llvm.org/D136947