net/mlx5: E-Switch, Have explicit API to delete fwd rules

Be symmetric with the e-switch API to add rules which has a
specific function to add fwd rules which are used as part of
vport mirroring.

This patch doesn't change any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>