Remove PageParallelJob as it is completely replaced now. It served us
well.

Bug: chromium:651354
Change-Id: I620fa7bb0dcaf7cd3710492913c501bcb162c781
Reviewed-on: https://chromium-review.googlesource.com/544950Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46159}