controller: l2_multicast_group_leave: use erase return value
controller: l2_multicast_group_leave: use erase return value

Instead of searching for a group id and then deleting the iterator, just
directly delete it and use the return value of ::erase to determine if
the group existed in the set.

Signed-off-by: Jonas Gorski <9712c064bde5a52658fc87d6454b4956f9bf286b@bisdn.de>

bisdn/basebox,bisdn/basebox,bisdn/basebox,bisdn/basebox