enable-av1-decoder: remove flag

Remove AV1 decoder flag from chrome://flags and checkout_libaom from
DEPS.

AV1 support has reached the stable branch. It is no longer possible to
build AV1 but not enable it. This simplifies many of the tests.

Keep ENABLE_AV1_DECODER build flag so we can restrict which
platforms support AV1.

Change-Id: I17de1f19cf58c3855667b441030ba747dbfd162b
Reviewed-on: https://chromium-review.googlesource.com/c/1341968
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#609459}
15 files changed