More WikitextSerializer cleanup and link-free image rt support

* Add support for the serialization of images with an empty link parameter.

* Remove the remaining TokenCollector use and rip out TokenCollector and
  related code.

Overall four more parser tests are passing with this patch vs. current master.

Change-Id: I07d28a056b4bb0086dc3f8e5acdd4083d77ccc9a