[keyserver] Update deleteAccount to handle user image avatars

Summary:
I looked through all of the DELETE queries in keyserver to see where we're handling the deletion of users. I found one query (deleteAccount) that needed to be updated to handle deletion of user image avatar (AKA upload with container=deletedUserID)

Depends on D7377

Test Plan:

1. Set image avatar for user.
2. Observe that container is set correctly in uploads table.
3. Delete user.
4. Observe that row in uploads table corresponding to user's image avatar has been removed.

This is safe because uploads has an index on container

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7379