Delete empty group upon last LEAVE operation

Currently when users join the channel they create buckets, if the users leave the group hangs around with 0 buckets . The mcast app needs to remove the remove the next objective referring to the group and call remove group in ONOS during removal of the laster subscriber in that group.

To reproduce the bug:

• bring up a voltha+onos
• create and enable olt
• push attached .json configuration with curl --fail -sSL --user karaf:karaf -X POST http://127.0.0.1:8181/onos/v1/network/configuration/apps/org.opencord.igmpproxy -H Content-type:application/json -d@mcastconfig.json
• ssh into bbsim with kubectl -n voltha exec -it $(kubectl -n voltha get pods -l app=bbsim -o name) – /bin/sh
• bbsimctl onu igmp BBSM00000001 join
• see installed group with 1 bucket in onos via `groups` command
  id=0x3, state=ADDED, type=ALL, bytes=0, packets=0, appId=org.opencord.mcast, referenceCount=0
  id=0x3, bucket=1, bytes=0, packets=0, weight=-1, actions=[OUTPUT:16]
• bbsimctl onu igmp BBSM00000001 leave
• see that the group is still present but with no bucket
  id=0x3, state=ADDED, type=ALL, bytes=0, packets=0, appId=org.opencord.mcast, referenceCount=0

This group needs to be removed the the last next objective referring to it is removed.