[Scheduling API] Remove experimental main-thread TaskQueue API

This experimental TaskQueue API is not currently under development or
experimentation and is being replaced by a new scheduler.postTask() API.
We're clearing out the old one to make room for and avoid name
collisions with the new one, and they are different enough to warrant a
clean slate.

Bug: 979017
Change-Id: I0ad25ff27acb819be36dba692bdc22fc1146db47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679068
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674155}