Remove allowDisablingImmediateExecutionOfScheduleMountItems feature flag

Summary:
No longer needed.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D20747684

fbshipit-source-id: a8077519b7670d72e23267b1c1423556ec97be3f

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -825,11 +825,6 @@ public class UIManagerModule extends ReactContextBaseJavaModule
     }
   }
 
-  @Override
-  public void setAllowImmediateUIOperationExecution(boolean flag) {
-    // Noop outside of Fabric, call directly on FabricUIManager
-  }
-
   /**
    * Schedule a block to be executed on the UI thread. Useful if you need to execute view logic
    * after all currently queued view updates have completed.