gen_stub: remove impossible Type::__toString() handling

`Type::$types` is typed as an array, and thus can never be `null`. There is no
need to have code to handle such a case.

Author: DanielEScherzer

build/gen_stub.php

@@ -726,10 +726,6 @@ public static function equals(?Type $a, ?Type $b): bool {
     }
 
     public function __toString() {
-        if ($this->types === null) {
-            return 'mixed';
-        }
-
         $char = $this->isIntersection ? '&' : '|';
         return implode($char, array_map(
             function ($type) { return $type->name; },