Use metadata API in do_all_pings() to delete post meta (cleaner, plays better...

Use metadata API in do_all_pings() to delete post meta (cleaner, plays better with cache). Remove redundant query to retrieve meta_id. See #18196.


git-svn-id: https://core.svn.wordpress.org/trunk@18855 1a063a9b-81f0-0310-95a4-ce76da25c4cd