Improve some docs for @param. Remove an unneeded $wpdb global import.

See #30799.


git-svn-id: https://develop.svn.wordpress.org/trunk@30979 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/meta.php

@@ -1146,8 +1146,6 @@ class WP_Meta_Query {
 	 * }
 	 */
 	public function get_sql( $type, $primary_table, $primary_id_column, $context = null ) {
-		global $wpdb;
-
 		if ( ! $meta_table = _get_meta_table( $type ) ) {
 			return false;
 		}