Eliminate use of `extract()` in `wp_xmlrpc_server::mw_editPost()` (MetaWeblog API, y'all). 

A lot of the extracted variables are overwritten by being explicitly set later. 
Only set variables that would otherwise not be present with `compact()` is called.

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28448 602fd350-edb4-49c9-b593-d223f7449a82