Bluetooth: discard bt_sock_unregister() errors

After we successfully registered a socket via bt_sock_register() there is
no reason to ever check the return code of bt_sock_unregister(). If
bt_sock_unregister() fails, it means the socket _is_ already unregistered
so we have what we want, don't we?

Also, to get bt_sock_unregister() to fail, another part of the kernel has
to unregister _our_ socket. This is sooo _wrong_ that it will break way
earlier than when we unregister our socket.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>