[CrOS MultiDevice] Remove remaining kMultiDeviceApi flags

Removing remaining kMultiDevice flags and associated code paths that
will no longer occur.

Bug: 912743
Change-Id: I169c667c22a9063668b7935ee87cc3f04b168df1
Reviewed-on: https://chromium-review.googlesource.com/c/1372464
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616440}
14 files changed