Pass activity object to the 'bp_activity_user_can_delete' filter.

This will allow plugins to do finer-grained checks on other parameters
such as the 'item_id' or 'secondary_item_id' to determine if the
activity item should be deleted.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7467 cdf35c40-ae34-48e0-9cc9-0c9da1808c22