Bluetooth: change bt_sock_unregister() to return void

There is no reason a caller ever wants to check the return type of this
call. _Iff_ a user successfully called bt_sock_register(), they're allowed
to call bt_sock_unregister().
All other calls in the kernel (device_del, device_unregister, kfree(), ..)
that are logically equivalent return void. Lets not make callers think
they have to check the return type of this call and instead simply return
void.

We guarantee that after bt_sock_unregister() is called, the socket type
_is_ unregistered. If that is not what the caller wants, they're using the
wrong function, anyway.

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