[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