[libc++] Revert "[SystemZ][z/OS] Missing wchar functions libc++"

This reverts commit febbf68b423b14d55a14980d2ba3ec37ef1e31dc because it
added files that were not under the LLVM license.

See https://reviews.llvm.org/D98207 for details.

Author: ldionne

libcxx/include/wchar.h

@@ -170,13 +170,13 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
 }
 #endif
 
-#if defined(__cplusplus) && (defined(_LIBCPP_MSVCRT_LIKE) || defined(__MVS__))
+#if defined(__cplusplus) && defined(_LIBCPP_MSVCRT_LIKE)
 extern "C" {
 size_t mbsnrtowcs(wchar_t *__restrict dst, const char **__restrict src,
                   size_t nmc, size_t len, mbstate_t *__restrict ps);
 size_t wcsnrtombs(char *__restrict dst, const wchar_t **__restrict src,
                   size_t nwc, size_t len, mbstate_t *__restrict ps);
-}  // extern "C"
-#endif  // __cplusplus && (_LIBCPP_MSVCRT || __MVS__)
+}  // extern "C++"
+#endif  // __cplusplus && _LIBCPP_MSVCRT
 
 #endif  // _LIBCPP_WCHAR_H

libcxx/src/CMakeLists.txt

@@ -92,8 +92,6 @@ elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "SunOS")
     )
 elseif(ZOS)
   list(APPEND LIBCXX_SOURCES
-    support/ibm/mbsnrtowcs.inc
-    support/ibm/wcsnrtombs.inc
     support/ibm/xlocale_zos.cpp
     )
 endif()