[native] Replace removeThreadAvatar with generic setThreadAvatar

Summary:
Decided to do the same thing for thread avatars here as I did for user avatars in D8017. It's good to unify codepaths, and this diff will allow me to deprecate useSaveThreadAvatar in the next diff, replacing it with the shared utilities in EditThreadAvatarProvider.

Depends on D8019

Test Plan: Test clearing thread avatar via iOS simulator

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8020