Site Health: Remove unused $wpdb global in utf8mb4 support test.

Follow-up to [44984], [56475].

Props upadalavipul, mukesh27.
Fixes #59977.

git-svn-id: https://develop.svn.wordpress.org/trunk@57141 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-site-health.php

@@ -1288,13 +1288,9 @@ class WP_Site_Health {
 	 *
 	 * @since 5.2.0
 	 *
-	 * @global wpdb $wpdb WordPress database abstraction object.
-	 *
 	 * @return array The test results.
 	 */
 	public function get_test_utf8mb4_support() {
-		global $wpdb;
-
 		if ( ! $this->mysql_server_version ) {
 			$this->prepare_sql_data();
 		}