remove incorrect __unused modifier

Summary:
the `update` argument was used below
Closes https://github.com/facebook/react-native/pull/9753

Differential Revision: D3820510

fbshipit-source-id: 8cad707a9cd4887c7ee07fe960b1f918761e64c5

React/Modules/RCTTiming.m

@@ -175,7 +175,7 @@ RCT_EXPORT_MODULE()
   return _sendIdleEvents || _timers.count > 0;
 }
 
-- (void)didUpdateFrame:(__unused RCTFrameUpdate *)update
+- (void)didUpdateFrame:(RCTFrameUpdate *)update
 {
   NSDate *nextScheduledTarget = [NSDate distantFuture];
   NSMutableArray<NSNumber *> *timersToCall = [NSMutableArray new];