DeviceSync: Clean up ShouldUseV1(V2)DeviceSync(), part 4

Delete methods that have DCHECK(features::ShouldUseV1DeviceSync())
and some related histograms.

There should be no behavior changes from this CL.

OBSOLETE_HISTOGRAM[MultiDevice.DeviceSyncService.FindEligibleDevices.Result]=Was used by a completed migration
OBSOLETE_HISTOGRAM[MultiDevice.DeviceSyncService.FindEligibleDevices.Result.FailureReason]=Was used by a completed migration

Bug: b:365057260
Change-Id: I2af93e4b73b071a364daf1375ef346c579a5ea20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5971676
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/main@{#1378361}