[web] turn off dummy avatar feature flag

Summary:
flip off avatar feature flag so that avatars now render on web. In ENG-3413 we consider removing and deprecating the feature flag rendering logic throughout our codebase which would be useful to do in the future since this flag is now always hardcoded to true

Depends on D7639

Test Plan: avatars now render on web

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7640