Revert "minor #40768 [HttpClient] [PHPDoc] Fix 2 remaining return mixed (fancyweb)"

This reverts commit 40d73152027fc63f2b63beaddb0fc454af8971c1, reversing
changes made to 4e904ec1081ef7888acc2e8277e55859ad275a25.

Author: nicolas-grekas

Response/HttplugPromise.php

@@ -49,8 +49,10 @@ public function getState(): string
 
     /**
      * {@inheritdoc}
+     *
+     * @return Psr7ResponseInterface|mixed
      */
-    public function wait($unwrap = true): ?Psr7ResponseInterface
+    public function wait($unwrap = true)
     {
         $result = $this->promise->wait($unwrap);