Remove objc_retainAutoreleasedReturnValue()

- This had been duplicated here and in swift-corelibs-foundation
  so consolidate it into the main Swift stdlib instead.

Author: spevans

src/swift/DispatchStubs.cc

@@ -198,13 +198,3 @@ SOURCE(TIMER)
 SOURCE(VNODE)
 #endif
 SOURCE(WRITE)
-
-// See comment in CFFuntime.c explaining why objc_retainAutoreleasedReturnValue is needed.
-extern "C" void swift_retain(void *);
-extern "C" void * objc_retainAutoreleasedReturnValue(void *obj) {
-    if (obj) {
-        swift_retain(obj);
-        return obj;
-    }
-    else return NULL;
-}