Removed a wrong assert.
Removed a wrong assert.

Summary:
This fixes [#15801](https://github.com/facebook/react-native/issues/15801)

We ran into a strange crash on iOS (debug only). After removing the clutter I was able to reproduce it in a tiny app. You can check it out [here.](https://github.com/simonracz/textinput_stress)

The UI in JS and native are not always in sync (which is okay). Due to this, a native view might call back into JS, which is no longer present in the shadow view hierarchy there. I think this should be also okay.

TextInput in some cases calls into [setIntrinsicContentView](https://github.com/facebook/react-native/blob/6d67e2dbbcd658b7f845ebb0d0156bd64dc68226/React/Modules/RCTUIManager.m#L382), where it triggers an overly enthusiastic `NSAssert` and crashes the app.

Check out [textinput_stress](https://github.com/simonracz/textinput_stress)
Rotate the simulator a few times to see the crash or the lack of crash.
Closes https://github.com/facebook/react-native/pull/16170

Differential Revision: D5959776

Pulled By: shergin

fbshipit-source-id: f39f5a3f1d86b330ecf7cbccd90871bc01fd69d9

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