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

[michaelplatings]

Summary

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.

[philnik]

LGTM if you make sure locally that it builds properly. We don't have a newlib configuration in the CI right now.

[michaelplatings]

Thanks Nikolas for the quick review

[michaelplatings]

We don't have a newlib configuration in the CI right now.

@philnik I'd therefore be interested to get your views on https://discourse.llvm.org/t/rfc-testing-of-newlib-picolibc/66509

libcxx/include/__support/newlib/xlocale.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
 #define _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
 
 #if defined(_NEWLIB_VERSION)
 
-#include <cstdlib>
-#include <clocale>
-#include <cwctype>
-#include <ctype.h>
 #if !defined(__NEWLIB__) || __NEWLIB__ < 2 || \
     __NEWLIB__ == 2 && __NEWLIB_MINOR__ < 5
 #include <__support/xlocale/__nop_locale_mgmt.h>
 #include <__support/xlocale/__posix_l_fallback.h>
 #include <__support/xlocale/__strtonum_fallback.h>
 #endif
 
 #endif // _NEWLIB_VERSION
 
 #endif