cld80211_lib: Make cld80211_remove_mcast_group empty function

cld80211_remove_mcast_group intents to drop from multicast
group membership when cld80211 cleanup.

However cld80211_remove_mcast_group is not mandatory for
cleanup, and it has risk to cause thread hang if driver
stopped prior to cleanup.

Ideally cld80211_remove_mcast_group can be removed from
functional perspective, but it is public function and
widely used by other modules, so make it an empty function
to keep compiler happy.

Change-Id: Ib1ab18e9aa1c166faa039853f269e75ac7470608
CRs-Fixed: 2752447