AggregateGroupsSpecialPage: Do not show already selected groups

From the entity selector filter out groups that are already a part
of the aggregate groups.

This feature was in place, but was broken due to changes in
I896bf4682ac5281ebf4309fdb7445dcd8cb9a75a

Bug: T366479
Change-Id: I68cff354e16801e93830d48c7690643345099b7a
2 files changed