Exclude 'last_activity' and 'total_member_count' from the bp_groups item cache

These items are stored in groupmeta, and thus are cached independently.
Furthermore, including them in the group cache will require that cache to be
busted more frequently than necessary.

See #5407

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7980 cdf35c40-ae34-48e0-9cc9-0c9da1808c22