Remove HAS_JS_WEAK_OBJECTS definition (#4105)

## Summary

I noticed that `HAS_JS_WEAK_OBJECTS` is no longer used so it makes sense
to remove it from our codebase.

## Test plan

Just see if the Example and FabricExample apps compile.

Common/cpp/SharedItems/Shareables.h

@@ -10,8 +10,6 @@
 #include "RuntimeManager.h"
 #include "Scheduler.h"
 
-#define HAS_JS_WEAK_OBJECTS (JS_RUNTIME_HERMES || JS_RUNTIME_V8)
-
 using namespace facebook;
 
 namespace reanimated {