nfp: flower: remove unused neighbour cache

With the neighbour entries now stored in a dedicated table there
is no use to make use of the tunnel route cache anymore, so remove
this.

Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>