Remove ended nightshade-preview

Author: GrahamCampbell

lib/Github/Api/Repo.php

@@ -696,9 +696,6 @@ public function replaceTopics($username, $repository, array $topics)
      */
     public function transfer($username, $repository, $newOwner, $teamId = [])
     {
-        //This api is in preview mode, so set the correct accept-header
-        $this->acceptHeaderValue = 'application/vnd.github.nightshade-preview+json';
-
         return $this->post('/repos/'.rawurldecode($username).'/'.rawurldecode($repository).'/transfer', ['new_owner' => $newOwner, 'team_id' => $teamId]);
     }
 }