XMLRPC: Remove the "Backward Compatibility" code from the new api as we don't...

XMLRPC: Remove the "Backward Compatibility" code from the new api as we don't need to support this older format and it keeps the new api cleaner.

The new Posts API is designed so that you can fetch a post with get, update the data structure returned and submit that with edit and so we need to ensure to only include in get data in a single form - tags and categories are returned as terms.
See #18429


git-svn-id: https://core.svn.wordpress.org/trunk@20154 1a063a9b-81f0-0310-95a4-ce76da25c4cd