openvswitch: clean up unused function

commit 6b001e68 ("openvswitch: Use Geneve device.")

The commit above deleted the only call site of ovs_tunnel_route_lookup()
and now that function is not used any more. So let's delete the function
definition as well.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>