users: clarify confusing check for last owner.
users: clarify confusing check for last owner.

Conceptually, we're clearly intending to check whether the user we're
mutating is the last realm owner. The preexisting code was safe
because we've already checked that the target user is an owner, and
thus if we're the last owner, we're the target user.

zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip