tp_base_contact_list_groups_removed: use get_group_members rather than peeking at the group mixin
tp_base_contact_list_groups_removed: use get_group_members rather than peeking at the group mixin

Ideally, TpBaseContactList wouldn't store redundant information at all;
it'd just be a view onto the model provided by its subclass.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib