Move the unused RCTTimingCls module under RCT_REMOVE_LEGACY_ARCH

Summary:
Changelog: [INTERNAL]

RCTTiming is NOT used in the RN new architecture: #57081

And will be deleted in: #57037

This change removes it from the binary

Differential Revision: D113474826

Author: christophpurrer

packages/react-native/React/CoreModules/RCTTiming.mm

@@ -429,7 +429,9 @@ - (void)createTimerForNextFrame:(nonnull NSNumber *)callbackID
 
 @end
 
+#ifndef RCT_REMOVE_LEGACY_ARCH
 Class RCTTimingCls(void)
 {
   return RCTTiming.class;
 }
+#endif