Remove invalidate observer instead of re-adding observer in DeviceInfo module (#43737)

Summary:
Previous fix brings in https://github.com/facebook/react-native/pull/42396. Seems it's a mistake to re-add observer?
So let's remove it and also not `invalidate` method not be called twice.

## Changelog:

[IOS] [FIXED] - Remove invalidate observer instead of re-adding observer in DeviceInfo module

Pull Request resolved: https://github.com/facebook/react-native/pull/43737

Test Plan: Fix for https://github.com/facebook/react-native/issues/42120 also works.

Reviewed By: javache

Differential Revision: D55692219

Pulled By: cipolleschi

fbshipit-source-id: dba1ddc39a9f2611fc2b84fadf8c23827891379a