Remove dead `rejectImpossiblePolicy()` method

Summary: Ref T603. Apparently we made all policies possible at some point. Go us! This has no callsites.

Test Plan: `grep`, notice it's a private method

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7259