Remove BackgroundTrigger from background mode.

BackgroundTrigger never ended up being used so remove it.

Change-Id: I96ae083bfe60c893b7ddd97921d6432a69d207ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634129
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Sam McNally <sammc@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664132}