Remove unused forward declaration of RCTMountingManager (#43453)

Summary:
Pull Request resolved: #43453

The forward declaration for RCTMountingManager doesn't appear to be used anywhere. It can be deleted.

## Changelog

[Internal]

Reviewed By: NickGerleman

Differential Revision: D54692764

fbshipit-source-id: ba8206408fdd515387a0a6aff6a3e43c51221e57

packages/react-native/React/Fabric/RCTScheduler.h

@@ -21,8 +21,6 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-@class RCTMountingManager;
-
 /**
  * Exactly same semantic as `facebook::react::SchedulerDelegate`.
  */