Improve layout of test for cached invalid, then valid site details

* Rename method to `test_get_blog_details_when_site_does_not_exist()`.
* Always assume `MAX(blog_id)` is 1 and therefore always create a burner.
* Remove tests specific to `wpmu_delete_blog()` and cache, to be handled elsewhere.
* Remove extra asertions.

See #30080, #23405


git-svn-id: https://develop.svn.wordpress.org/trunk@30175 602fd350-edb4-49c9-b593-d223f7449a82
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


git-svn-id: https://develop.svn.wordpress.org/trunk@30176 602fd350-edb4-49c9-b593-d223f7449a82
Use common core syntax for checking DOING_AJAX

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

props TobiasBg
Fixes #29891


git-svn-id: https://develop.svn.wordpress.org/trunk@30177 602fd350-edb4-49c9-b593-d223f7449a82