[native] remove avatar feature flag logic

Summary: Now that avatars is in a stable spot, we can remove the return true feature flag function and the related logic to conditionally render avatars based on it

Test Plan: Made sure that as I was removing the feature flag code, the avatar rendering looked the same

Reviewers: atul, rohan

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8186