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


git-svn-id: https://develop.svn.wordpress.org/trunk@56475 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/utils.php

@@ -556,7 +556,6 @@ class WpdbExposedMethodsForTesting extends wpdb {
 	public function __construct() {
 		global $wpdb;
 		$this->dbh         = $wpdb->dbh;
-		$this->use_mysqli  = $wpdb->use_mysqli;
 		$this->is_mysql    = $wpdb->is_mysql;
 		$this->ready       = true;
 		$this->field_types = $wpdb->field_types;

tests/phpunit/tests/db.php

@@ -713,9 +713,6 @@ class Tests_DB extends WP_UnitTestCase {
 	 */
 	public function test_mysqli_flush_sync() {
 		global $wpdb;
-		if ( ! $wpdb->use_mysqli ) {
-			$this->markTestSkipped( 'mysqli not being used.' );
-		}
 
 		$suppress = $wpdb->suppress_errors( true );