Fixed crash happenned on deallocating RCTSurface

Summary:
Essentially, we had `dispatch_async` inside `delloc` method which tried to retain `self`, which is disallowed operation in ObjC runtime.
However, we don't need to notify anything `surface`-related in `_stop` because it always is called from `dealloc`.

Reviewed By: mmmulani

Differential Revision: D6665631

fbshipit-source-id: ed0d192946f3323f4f54ecb99b30e56e0942f174

React/Base/Surface/RCTSurface.mm

@@ -306,8 +306,6 @@
                         method:@"unmountApplicationComponentAtRootTag"
                           args:@[self->_rootViewTag]
                     completion:NULL];
-
-  [self _setStage:RCTSurfaceStageSurfaceDidStop];
 }
 
 - (void)_registerRootView