`utils/Digraph` had a bunch of code that wasn't actually being used,
much of which wasn't documented at all, some of which was clearly
ill-considered, and some of which was documented as being inefficient.

Remove all unused code from that module except for the obvious and
innocuous `emptyG`.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4676