Remove useOverflowInset flag as we rolled out 100% to public for over three months

Summary:
This was shipped in D36990986 (https://github.com/facebook/react-native/commit/df80ed40c7cde6cf81a0974d78ef0a7cfcf19e5c) but backed out last year in D37074879 (https://github.com/facebook/react-native/commit/59476d06f3be2e1c764c21447b4cf2ea712dd172), as we want to wait for some performance comparison results to come out.

Remove overflow inset optimization flags as they've been rolled out 100% to public.

Changelog:
[Android][Internal] - Clean up feature flags for overflowInset

Reviewed By: javache

Differential Revision: D43070494

fbshipit-source-id: dbf5aed9b2b5d3db1ad351bc208cb2016dc62e40