[native] Introduce useRemoveThreadAvatar hook

Summary:
We introduce the useRemoveThreadAvatar hook so we can unset the user avatar. This is effectively D7512 but for threads.

Depends on D7512

Test Plan:
Able to clear thread avatars:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7513