Delete activity comment meta when deleting parent activity items.

Previously, when deleting parent activity items, BP would delete the
related activity comments, but not the activity comment metadata.

To solve this, activity comments are now queried and removed in the
main BP_Activity_Activity::delete() method and the activity comment
IDs are now properly passed to the delete_activity_meta_entries()
method.

Commit also includes a unit test for this issue.

Hat-tip boonebgorges for feedback.

Fixes #5097.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7422 cdf35c40-ae34-48e0-9cc9-0c9da1808c22