remove unused methods from RCTAppearance (#41920)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41920

Changelog: [Internal]

these overrides do nothing, the superclass leaves these empty as well. remove

Reviewed By: cipolleschi

Differential Revision: D51968873

fbshipit-source-id: a214903d2834aa502c9e81ebaab928694bfa0818

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

@@ -133,14 +133,6 @@ RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSString *, getColorScheme)
   return @[ @"appearanceChanged" ];
 }
 
-- (void)startObserving
-{
-}
-
-- (void)stopObserving
-{
-}
-
 - (void)invalidate
 {
   [super invalidate];