[CrOS MultiDevice] Remove the kEnableUnifiedMultiDeviceSettings flag.

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

Bug: 912743
Change-Id: If13ad69ebad024f64d2bcfdcb9a9278af45cb766
Reviewed-on: https://chromium-review.googlesource.com/c/1381000
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@{#617325}
6 files changed