Remove irrelevant filter.js and filter.css includes

As per Drupal 6 Upgrade to 4.x warning: file_get_contents filter.js failed to open an attempt is made to include files from the Drupal core "filter" module that only exist in Drupal 7 and not Drupal 6.

This patch simply removes the attempt to include these files. This prevents warning messages about the missing files from appearing in the Drupal log.