Memory permission backend implementation of remove_principal() is wrong

[glasserc]

According to the PermissionBase docstring, remove_principal is supposed to Remove a principal from every user. In other words, remove_principal(principal) is equivalent to remove_user_principal(user_id, principal) for user_id in all_possible_user_ids. However, the current implementation stores all permissions of all kinds in one hash table, and removes the principal from permissions of non-user things as well.