remove useless 'xml-rpc engine' setting from blogapi module

moshe brought to my attention that the blogapi RSD was giving preferred treatment to bloggerapi - since we actually support much more feature rich apis (namely movabletype's) ... this patch gives the preferred treatment to MT. Also, it removes the unused "XML-RPC engine" setting - which wasn't being used anyway. And, since, any of the 3 API options will work regardless, this setting is extraneous.