Change Treasury CDN rule, removing /d/

This looks like a red herring in the redirector's source code which we copied.
Looking at the hit data, there are almost no hits for URLS of the form
cdn.hm-treasury.gov.uk/d/file.pdf I suspect that the handful that exist are us
manually testing to see what redirector does with them. On the other hand,
there is some traffic of the form /file.pdf, so lets handle that.

From looking at the hit data for www.hm-treasury.gov.uk, there are some URLs of
the form www.hm-treasury.gov.uk/d/document.pdf and also
www.hm-treasury.gov.uk/another-document.pdf. I suspect that this led to a
misunderstanding.

Hit data:
https://github.com/alphagov/transition-stats