Clean up code related to experiment that delays reqeuests on multiplexed connections

This experiment has been enabled for ~6 months from
https://chromium-review.googlesource.com/c/chromium/src/+/1257677.
This CL cleans up the related field trial code.

Change-Id: Ic7a50e8e8b47b723a492d51a2b8ed841f898e416
Bug: 836552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576203
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652922}
5 files changed