bridge: mcast: Merge the mc router ports deletions to one function

There are three places where a port gets deleted from the mc router port
list. This patch join the actual deletion to one function.
It will be helpful for later patch that will offload changes in the mc
router ports list.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>