CHECK in `TurnSyncOnHelper` to restrict is used if feature enabled

This change adds a `CHECK` to `TurnSyncOnHelper`'s constructor to ensure
it's not exercised when `syncer::kReplaceSyncPromosWithSignInPromos` is
enabled.

It also removes TODOs in tests that suggested the feature override
should be revisited, because it is deemed legit.

Fixed: 444621074
Change-Id: Ie2ef79619ee913bfaea76e1fa91544b9fa241082
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6980327
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520520}