network: Remove connman_network_register/unregister()

network: Remove connman_network_register/unregister()

Since the service object doesn't take a reference the
intermediate step of having a connman_network_register/unregister
is not needed. This function are only there to have an additional
step where probe()/remove() will be called. This is the job of
connman_device_add_network()/connman_device_remove_network().