Formats: Remove display & related method (#2776)

Format should not be responsible for sending HTTP response.

formats/AtomFormat.php

@@ -156,14 +156,6 @@ EOD;
 		return $toReturn;
 	}
 
-	public function display(){
-		$this
-			->setContentType(self::MIME_TYPE . '; charset=' . $this->getCharset())
-			->callContentType();
-
-		return parent::display();
-	}
-
 	private function xml_encode($text){
 		return htmlspecialchars($text, ENT_XML1);
 	}