clean up shouldUpdate method on RenderUnit

Summary: Now that we can granularly update the tree via the tracking in`SignalsMountState`, there's no need for the old approach

Reviewed By: rooju

Differential Revision: D62677578

fbshipit-source-id: c82b98e8f938833d23b9fd5d718c41d7abed8b50

litho-rendercore/src/main/java/com/facebook/rendercore/RenderUnit.kt

@@ -647,8 +647,6 @@ constructor(
     }
   }
 
-  open fun shouldUpdate(newRenderUnit: RenderUnit<MOUNT_CONTENT>): Boolean = this !== newRenderUnit
-
   open fun <T : Binder<*, *, *>?> findAttachBinderByClass(klass: Class<T>): T? {
     return attachBinderTypeToDelegateMap?.get(klass)?.binder as T?
   }