Remove Background Fetch' scheduler's ability to parallelize

We're not using this code in production. It's hardly tested, has an odd
sequencing order and brings a lot of complication that we can remove for
now. It's also the cause of a number of crashes.

Bug: 850512
Change-Id: I33050f54dfbafa9192390d1ad5f3ed34cce91aca
Reviewed-on: https://chromium-review.googlesource.com/1165554
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582192}
14 files changed