Formats: Remove display & related method (#2776)

Format should not be responsible for sending HTTP response.

formats/PlaintextFormat.php

@@ -21,12 +21,4 @@ class PlaintextFormat extends FormatAbstract {
 		$toReturn = mb_convert_encoding($toReturn, $this->getCharset(), 'UTF-8');
 		return $toReturn;
 	}
-
-	public function display(){
-		$this
-			->setContentType(self::MIME_TYPE . '; charset=' . $this->getCharset())
-			->callContentType();
-
-		return parent::display();
-	}
 }