Remove 188 Letterhead generation

* This is a very particular, peculiar use case that doesn't fit into any
  of the rest of the certificates workflow (and indeed doesn't seem to
  use any of it), so kill it.

I think this code is unused. I *know* it's unused by Stanford and the rest of the open source community. I think it may be unused by edx.org too.

@jarv if he has time, otherwise not sure who is good to review.