Remove Dependency Links - Needs Discussion

So when dependency links were deprecated we specified they'd have an accelerated removal and they'd be removed in 1.6. We are now at the point that develop is 1.6.dev1 so I've done the work to remove these. However the point stands of do we want to remove these.

There have been a few requests to keep them:

- https://www.mail-archive.com/distutils-sig@python.org/msg18983.html
- I could have swore that there was an issue in our tracker, however i can't locate it now

Overall I think that this is a misfeature of setuptools that pip should no longer support. There doesn't seem to be a massive number of users for this feature, and nobody has thus far pointed out any uses of dependency links that can't also be handled perfectly fine with a requirements.txt file.

What do we think?