iOS - deprecate iOS 9 support by removing runtime checks for 10.0+

Summary:
This removes all callsites that rely on runtime checks to detect the target deployment version. We no longer need to check for iOS 10+ in a few places. Note: for this to compile, the hosting app needs to target iOS 10.0+.

Changelog: [iOS] [Deprecated] - Deprecate iOS 9

Reviewed By: hramos

Differential Revision: D19271321

fbshipit-source-id: 424ad7e2161261d148cb436cc20b4c531a4ba5b7