Revert "debian/watch: Add back pgpsigurlmangle."

Revert "debian/watch: Add back pgpsigurlmangle."

I have to check what uscan is doing that it is using downloadurlmangle and
making the signature file be missed with:

,----[ gbp import-orig --pristine-tar --uscan ]
|
| gbp:info: Launching uscan...
| uscan: Newest version of youtube-dl on remote site is 2016.08.17, local version is 2016.06.25
| uscan:    => Newer package available from
|       https://github.com/rg3/youtube-dl/archive/2016.08.17.tar.gz
| uscan warn: In directory ., downloading
|   https://github.com/rg3/youtube-dl/archive/2016.08.17.tar.gz.sig failed: 404 Not Found
| uscan warn: FAIL Checking OpenPGP signature (no signature file downloaded).
| gbp:error: Couldn't find tarball at '../2016.08.17.tar.gz'
`----

This reverts commit cdea2e429e5bfc70c13b9a15efd0be9936703147.