In the groups dashboard, fix bulk deletion.  Fixes #5105.

* Use bp_admin_list_table_current_bulk_action() to reconcile what the
  proper action is when doing a bulk action like deleting.
* Fix copypasta from activity admin dashboard to use group IDs.

Props imath.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7360 cdf35c40-ae34-48e0-9cc9-0c9da1808c22