Revert "Add linux-bionic mixin, update webrtc waterfall"

This reverts commit 71994709.

Reason for revert: The general chromium swarming pool still runs 16.04 hence I need to revert this until they have been upgraded.

Original change's description:
> Add linux-bionic mixin, update webrtc waterfall
>
> Bug: 1183266
> Change-Id: Ifcaefed58b0b5a0c7e75d0993aa891327d6629f7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2731769


> Commit-Queue: Christoffer Jansson <jansson@chromium.org>
> Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#859742}

Bug: 1183266
Change-Id: Iaf8aae0bc61161b21cb73675a4a556c754e37c63
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2739574
Auto-Submit: Christoffer Jansson <jansson@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#860165}