Summary:
[After we migrated to new GIF implementation](https://github.com/facebook/react-native/pull/24822), we can remove old implementation now.
## Changelog
[iOS] [Deprecated] - Remove old GIF code
Pull Request resolved: https://github.com/facebook/react-native/pull/25636
Test Plan: GIF still works.
Reviewed By: shergin
Differential Revision: D16280044
Pulled By: osdnk
fbshipit-source-id: 00979280e6c17c93859ce886dd563b0d185c84aa