minor #5771 Remove not existing response constant (amansilla)

This PR was merged into the 2.3 branch.

Discussion
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Remove not existing response constant

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?  | no
| Applies to    | 2.3
| Fixed tickets | #5770

Commits
-------

b38c36c Remove not existing response constant

book/testing.rst

@@ -273,7 +273,7 @@ document::
         $this->assertTrue($client->getResponse()->isNotFound());
         // Assert a specific 200 status code
         $this->assertEquals(
-            200, // or Symfony\Component\HttpFoundation\Response::HTTP_OK
+            200,
             $client->getResponse()->getStatusCode()
         );