net: websocket: Add a way to unregister a websocket connection

Doing a normal close for a websocket does not close the underlying
real socket. If we do not have fd for the real socket, then it is
not possible to fully close a websocket connection. As we are allocating
a websocket using websocket_register() in HTTP server use case,
create a websocket_unregister() that will close both the real
socket and the websocket socket.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>