[iOS] Remove unused detector method (#4226)

## Description

Removes unused `shouldAttachGestureToSubview` declaration

## Test plan

Build any of the apps

Author: j-piasecki

packages/react-native-gesture-handler/apple/RNGestureHandlerDetector.h

@@ -33,8 +33,6 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)detachNativeGestureHandlers;
 
-- (BOOL)shouldAttachGestureToSubview:(nonnull NSNumber *)handlerTag;
-
 @end
 
 NS_ASSUME_NONNULL_END