Properly deprecated deleted function bp_activity_get_sitewide()

Deleted in 1.2, this function was still called in
BP_Activity_Activity::get_sitewide_items_for_feed().

Fixes #5023

Props ericlewis

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7201 cdf35c40-ae34-48e0-9cc9-0c9da1808c22