Remove support for the mysql extension

[johnbillion]

The mysql extension is no longer used in PHP 7 or above. There's a good amount of conditional code in wpdb that can be removed now that only the mysqli functions are used.

It's unlikely there will be a noticeable performance gain from the removal of the dead conditional code, but every little helps, and maintenance will be simplified.

Trac ticket:

Trac ticket:

@johnbillion The PR have list of PHPCS issues that need to fix.

[Hareesh Pillai]

Adding the needs-testing keyword.

On having a cursory glance, the changes look ok. This PR requires in-depth testing.

@mukeshpanchal27 Yeah they _should_ all be about indentation as mentioned in the PR description. I didn't fix the indentation in order to keep the diff clean and it can be addressed in the commit.

[johnbillion]

In 56475:

Database: Remove support for the mysql extension.

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

https://core.trac.wordpress.org/changeset/56475