Remove monkey_session fixture
Remove monkey_session fixture

The one user of monkey_session (map_config) was removed in PR #894, so
removing monkey_session as well. It appears the same functionality can
be accomplished by using the monkeypatch fixture in a derived fixture,
so we don't need to keep this implementation around just in case.

mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea