Make new_players GC when observing

When observing, that observer would be given their original new_player's mind, but that mind would still have the new_player as its current. This caused the new_player to fail to GC and resulted in the observer's mind's current becoming null.

Fix: Just delete the new_player mind when the player observes. If an admin turns them into a human or whatever, a new mind will be created automatically then.
Merge pull request #4340 from MrPerson/giacom_was_right_I_should_have_just_deld_mobs

Make new_players dereference from their mind when observing