selftests: bridge_igmp: Remove unnecessary address deletion

The test group address is added and removed in v2reportleave_test().
There is no need to delete it again during cleanup as it results in the
following error message:

 # bash -x ./bridge_igmp.sh
 [...]
 + cleanup
 + pre_cleanup
 [...]
 + ip address del dev swp4 239.10.10.10/32
 RTNETLINK answers: Cannot assign requested address
 + h2_destroy

Solve by removing the unnecessary address deletion.

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