Introduce friends_friendship_post_delete hook

The existing hook friends_friendship_deleted hook actually fires before the
friendship count is deleted in the database. This causes problems for plugins
that want to access the updated user friend counts, and other info that is
changed at the time of delete.

Fixes #5019

Props mukkundthanki

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7108 cdf35c40-ae34-48e0-9cc9-0c9da1808c22