Fix issues of unused code detected by scrutinizer

src/PassThroughRequestHandler.php

@@ -32,7 +32,6 @@ public function handle(ServerRequestInterface $request): ResponseInterface
         $next = $this->next;
         /** @var Generator $handlerGenerator */
         $handlerGenerator = $next($request);
-        $next = null;
 
         return new PromiseResponse($handlerGenerator->current());
     }

src/Runner/ReactRequestHandlerRunner.php

@@ -23,26 +23,6 @@ class ReactRequestHandlerRunner extends RequestHandlerRunner
      */
     private $handler;
 
-    /**
-     * A factory capable of generating an error response in the scenario that
-     * the $serverRequestFactory raises an exception during generation of the
-     * request instance.
-     *
-     * The factory will receive the Throwable or Exception that caused the error,
-     * and must return a Psr\Http\Message\ResponseInterface instance.
-     *
-     * @var callable
-     */
-    private $serverRequestErrorResponseGenerator;
-
-    /**
-     * A factory capable of generating a Psr\Http\Message\ServerRequestInterface instance.
-     * The factory will not receive any arguments.
-     *
-     * @var callable
-     */
-    private $serverRequestFactory;
-
     /**
      * React Http Server
      *