The `mysql` extension is no longer used in PHP 7 or above. There's a good amount of conditional code in `wpdb` and the health checks that can be removed now that only the `mysqli` functions are used.
Fixes#59118
git-svn-id: https://develop.svn.wordpress.org/trunk@56475 602fd350-edb4-49c9-b593-d223f7449a82