[Behat] Remove not used property

Author: Miguel

Features/Context/PlatformUI.php

@@ -72,11 +72,6 @@ class PlatformUI extends RawMinkContext
      */
     protected $password = 'publish';
 
-    /**
-     * Mapping of pages URL's.
-     */
-    protected $pageIdentifierMap = array();
-
     /**
      * Stores the status of the platform.
      * @var int