[native] Introduce useRemoveUserAvatar hook

Summary:
We introduce the useRemoveUserAvatar hook so we can unset the user avatar. Will introduce something similar for threads momentarily.

Depends on D7511

Test Plan:
Able to clear user avatar:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7512