make releasing less convoluted

SQL/migration.abstract.class.php

@@ -89,8 +89,6 @@
 			self::$sql = SQL::FromDSN($dsn);
 		}
 		final public static function release(){
-			$sql = self::$sql;
-			unset($sql);
 			self::$sql = null;
 		}
 	}