General: Restore usage of $wpdb, instead of $this->db.

Hiding the `$wpdb` global behind a property decreases the readability of the code, as well as causing irrelevant output when dumping an object.

Reverts [38275], [38278], [38279], [38280], [38387].
See #37699.



git-svn-id: https://develop.svn.wordpress.org/trunk@38768 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user.php

@@ -181,9 +181,6 @@ class Tests_User extends WP_UnitTestCase {
 		$this->assertEquals( 'foo', $user->data->$key );  // This will fail with WP < 3.3
 
 		foreach ( (array) $user as $key => $value ) {
-			if ( $value instanceof wpdb ) {
-				continue;
-			}
 			$this->assertEquals( $value, $user->$key );
 		}
 	}