devlink: Stop reference counting packet trap groups

Now that drivers explicitly register their supported packet trap groups
there is no for devlink to create them on-demand and destroy them when
their reference count reaches zero.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>