By default, BP_Activity_Activity::get_registered_components() should not return components with only last_activity items

Since moving last_activity items to be stored in the activity database, the
'members' component now has activity associated with it. However, these items
are not "real" activity items, in the sense that you would want to query them
when pulling up an activity stream. BP core has excluded these items from
general activity queries elsewhere. But in BP_Activity_Activity::get_registered_components(),
we continued to return the members component, even when the only activity
items associated with it were of the 'last_activity' variety. Though BP does
not natively use this method anymore, it did break some plugins that were
expecting a more sanitized list of components.

Fixes #5519

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8249 cdf35c40-ae34-48e0-9cc9-0c9da1808c22