[CrOS MultiDevice] Remove kEnableUnifiedMultiDevice from settings.

This CL removes kEnableUnifiedMultiDevice as well as its JS
representation (the loadTimeData "enableMultideviceSettings" property)
from settings. The feature has already been rolled out, and the flag is
no longer necessary.

Bug: 912743
Change-Id: Ie729fb732f83f07762be30cb5b0c3ce588905a3a
Reviewed-on: https://chromium-review.googlesource.com/c/1380949
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617317}
9 files changed