Disallow object:remove() if the object is a player

For example, this fixes the following error reported by VanessaE, using worldedit:

1. define an area using //pos1 and //pos2
1. position yourself inside this area and run //clearobjects

Without the patch, this will delete your player object, emit an error message like
<code>ERROR[ServerThread]: Server::ProcessData(): Cancelling: No player for peer_id=2 disconnecting peer!</code>
and you will then be disconnected.