Delete admin_created_user_subject() rather than deprecate

As it was never used as anything more than a callback to a filter before the MU merge, and is only available in user-new.php in multisite, it is safe to remove this function entirely.

Fixes #29915

Built from https://develop.svn.wordpress.org/trunk@30176


git-svn-id: http://core.svn.wordpress.org/trunk@30176 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use common core syntax for checking DOING_AJAX

Our current check for a non truthy `DOING_AJAX` is `! DOING_AJAX`.

props TobiasBg
Fixes #29891

Built from https://develop.svn.wordpress.org/trunk@30177


git-svn-id: http://core.svn.wordpress.org/trunk@30177 1a063a9b-81f0-0310-95a4-ce76da25c4cd