Removes duplicate Linux, Win and Mac testing.
These are superceded by the standalone bots now.
Android is still entirely Chromium-based.

Bug: angleproject:5114
Change-Id: I745758547b2eaa5f82779b11e7f3a8512c3cdd94
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2872072Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>