Cut old memrchr port in global namespace

Summary:
[Folly] Cut old memrchr port in global namespace since it is excessively difficult to make it portable across all platforms.

Fixes: https://github.com/facebook/folly/issues/817.
Fixes: https://github.com/facebook/folly/issues/1391.

Reviewed By: Orvid

Differential Revision: D22130039

fbshipit-source-id: 43511aa95c749d5f603ff8043c438ba63d44f6c1

folly/portability/String.cpp

@@ -16,17 +16,6 @@
 
 #include <folly/portability/String.h>
 
-#if !FOLLY_HAVE_MEMRCHR
-extern "C" void* memrchr(const void* s, int c, size_t n) {
-  for (auto p = ((const char*)s) + n - 1; p >= (const char*)s; p--) {
-    if (*p == (char)c) {
-      return (void*)p;
-    }
-  }
-  return nullptr;
-}
-#endif
-
 #if defined(_WIN32) || defined(__FreeBSD__)
 extern "C" char* strndup(const char* a, size_t len) {
   auto neededLen = strlen(a);

folly/portability/String.h

@@ -25,10 +25,6 @@
 #include <strings.h>
 #endif
 
-#if !FOLLY_HAVE_MEMRCHR
-extern "C" void* memrchr(const void* s, int c, size_t n);
-#endif
-
 #if defined(_WIN32) || defined(__FreeBSD__)
 extern "C" char* strndup(const char* a, size_t len);
 #endif