bp-legacy: When bulk-deleting message threads, use messages_delete_thread().

Previously, a foreach loop and a call to BP_Messages_Thread::delete()
was used to bulk-delete message threads from the inbox with AJAX.

This commit:
- Gets rid of the foreach loop and replaces the direct BP_Messages_Thread::delete() call with BP's messages_delete_thread() function.
- Properly sanitizes the posted 'thread_ids' with wp_parse_id_list().

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8052 cdf35c40-ae34-48e0-9cc9-0c9da1808c22