udp_tunnel: remove REGISTER/UNREGISTER handling from tunnel drivers

udp_tunnel_nic handles REGISTER and UNREGISTER event, now that all
drivers use that infra we can drop the event handling in the tunnel
drivers.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>