[libc++] Remove obsolete Solaris and Newlib support for locales (#113721)

The solaris header file doesn't even exist, so that's definitely dead
code. The newlib header is empty, which means that localization can't
work on that platform. If someone is using libc++ with Newlib, they must
be providing LIBCXX_HAS_NO_LOCALIZATION today for anything to work, so
that header is basically dead code as well.

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -497,7 +497,6 @@ set(files
   __locale_dir/locale_base_api/fuchsia.h
   __locale_dir/locale_base_api/ibm.h
   __locale_dir/locale_base_api/musl.h
-  __locale_dir/locale_base_api/newlib.h
   __locale_dir/locale_base_api/openbsd.h
   __locale_dir/locale_base_api/win32.h
   __locale_dir/locale_guard.h

libcxx/include/__locale_dir/locale_base_api.h

@@ -15,10 +15,6 @@
 #  include <__locale_dir/locale_base_api/ibm.h>
 #elif defined(__ANDROID__)
 #  include <__locale_dir/locale_base_api/android.h>
-#elif defined(__sun__)
-#  include <__locale_dir/locale_base_api/solaris.h>
-#elif defined(_NEWLIB_VERSION)
-#  include <__locale_dir/locale_base_api/newlib.h>
 #elif defined(__OpenBSD__)
 #  include <__locale_dir/locale_base_api/openbsd.h>
 #elif defined(__Fuchsia__)

libcxx/include/__locale_dir/locale_base_api/newlib.h

@@ -1451,7 +1451,6 @@ module std [system] {
       textual header "__locale_dir/locale_base_api/fuchsia.h"
       textual header "__locale_dir/locale_base_api/ibm.h"
       textual header "__locale_dir/locale_base_api/musl.h"
-      textual header "__locale_dir/locale_base_api/newlib.h"
       textual header "__locale_dir/locale_base_api/openbsd.h"
       textual header "__locale_dir/locale_base_api/win32.h"
     }