Remove repository shortcuts

Summary:
Repositories currently have a no-UI "shortcut" feature which is only used by Facebook (and I'm not sure it's even used). As implemented, this feature is policy-oblivious and kind of nonsensical. Throw it away.

I'm open to reimplementing this, but I want to see some level of interest in it before I do. The new implementation would add shortcuts to each repository, similar to how mirrors work. My original plan was to follow this up with such an implementation (it's half-implemented in my sandbox), but as I worked through it I'm not sure it's really valuable.

Test Plan: Browsed repository list, grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: FacebookPOC, aran

Differential Revision: https://secure.phabricator.com/D7862