selftests: bridge_vlan_mcast: Delete qdiscs during cleanup

The qdiscs are added during setup, but not deleted during cleanup,
resulting in the following error messages:

 # ./bridge_vlan_mcast.sh
 [...]
 # ./bridge_vlan_mcast.sh
 Error: Exclusivity flag on, cannot modify.
 Error: Exclusivity flag on, cannot modify.

Solve by deleting the qdiscs during cleanup.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>