[web] Add removeUserAvatar to EditUserAvatarMenu

Summary: Sets "Reset to default" menu item onClick to removeUserAvatar which removes the user's avatar.

Test Plan:
Works as expected. User had image avatar, now they have default emoji avatar:

Reviewers: ashoat, ginsu, rohan, kamil, inka, michal

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8260