[native] Fix removeEventListener deprecation warn

Summary:
In React Native 0.66, all removeEventListener() functions became deprecated in favor of subscription.remove() obtained by subscription = addEventListener().

In our app lifecycle listener we have already been following the new pattern, by creating our own facade for the EventSubscription. It is now no longer needed, as we can return the React Native subscription object directly.

I'm going to need this interface in my height measurement stuff, so I'm fixing this ahead of time.

Test Plan:

• Ran the native app and confirmed this warn doesn't show up anymore
• Confirmed with debugger that listener subscription is removed properly

Reviewers: ashoat, tomek, atul

Reviewed By: ashoat, tomek, atul

Subscribers: atul, abosh

Differential Revision: https://phab.comm.dev/D5386