Remove route for video part of guide

This functionality was removed a while ago.  with this route in place, a
MissingTemplateException is caused when hitting this route.  Removing
the route will make it behave like any other non-existent part of a
guide, and redirect to the start of the guide.
Merge pull request #284 from alphagov/remove_video_part_route

Remove route for video part of guide