[CrOS MultiDevice] Remove the kEnableUnifiedMultiDeviceSetup flag.

All references to the flag had already been removed, so this flag is no
longer needed.

Bug: 912743
Change-Id: I381a3a11821009b73ada65fccc8746851f6d6134
Reviewed-on: https://chromium-review.googlesource.com/c/1380998
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617585}
7 files changed