Remove wp-register.php.

 * Don't remove it on update. If you have it now, you'll keep it. New installs won't get it.
 * Peform a redirect using rewrite + canonical to the new location, if the file doesn't exist.
props SergeyBiryukov.
fixes #18544.


git-svn-id: https://core.svn.wordpress.org/trunk@20596 1a063a9b-81f0-0310-95a4-ce76da25c4cd