The cloner controls index build oplog entries by FCV added in SERVER-43323.

The IndexBuildsCoordinator decides whether to abort index builds by FCV added in SERVER-39002.

Downgrading FCV isn't allowed with active index builds added in SERVER-45944.

StorageEngineImpl::supportsTwoPhaseIndexBuild added in SERVER-44742. And the caller thereof.

Looks like jstests/multiVersion/index_initial_sync_latest_primary.js and jstests/multiVersion/index_initial_sync_latest_secondary.js can be deleted.

Please check the SERVER ticket(s) that added the FCV gates for anything else incidental to remove, particularly any testing that can be deleted.