selftests: forwarding: mirror_vlan: Uninstall trap

Instead of installing a trap before tests run and uninstalling it after
they run, mirror_vlan.sh installs it twice due to a typo. Fix the typo.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>