Revert "Fallback to old thresholds API as necessary (take 2)"

Authored by awight on Oct 11 2017, 8:44 PM.

Description

Revert "Fallback to old thresholds API as necessary (take 2)"

Do not deploy until all old thresholds servers are replaced.

Simplifies thresholds handling by dropping back-compatibility with revscoring 1.

This mutatedly reverts commit 451b61af4a5a2e00426d18820009466a1c033812.

Bug: T179602
Change-Id: I455859cf6877efef3da318bfcf0884ddf84964f1