Polish removal of conduit shield, including legacy stripping for phabricator on phabricator oauth scenarios

Summary: ...just in case that stuff happens in the "wild". also cleaned up the logic here since we no longer have the conduit conditionality.

Test Plan: made sure I didn't break JS on the site. reasoned about logic of my function and asking people PHP typing questions in job interviews.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T891

Differential Revision: https://secure.phabricator.com/D3269

src/aphront/response/AphrontJSONResponse.php

@@ -44,7 +44,7 @@ final class AphrontJSONResponse extends AphrontResponse {
   public function buildResponseString() {
     $response = $this->encodeJSONForHTTPResponse($this->content);
     if ($this->shouldAddJSONShield()) {
-      $response = $this->addJSONShield($response, $use_javelin_shield = false);
+      $response = $this->addJSONShield($response);
     }
     return $response;
   }