Introduce wp_remove_object_terms() and wp_add_object_terms(). No more fetch-alter-update nonsense. Use wp_remove_object_terms() in a few places internally.

props simonwheatley, scribu, ericmann, maxcutler. fixes #15475

git-svn-id: http://core.svn.wordpress.org/trunk@23398 1a063a9b-81f0-0310-95a4-ce76da25c4cd