Remove unreachable DifferentialRevisionStatsView

Summary:
Ref T2222. I wiped out the Differential-specific stats page a long time ago, but missed this. It turned up recently in `grep`.

Facts will eventually fill this role; this code is unreachable; it probably doesn't work now and definitely won't work in a day or two after ApplicationTransactions.

Test Plan: Used `grep` to look for callsites.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8208