Create a no-op scheduler job for mac-osxbeta-rel.

mac-osxbeta-rel was removed from the branches because it is an FYI
builder that is not the mirror of any try builders and is red on the
branches so it is a distraction for sheriffs. Removing it from the
branches causes 'Mac Builder' to fail on the branches when attempting to
trigger it. The no-op scheduler job will allow the trigger to succeed.

Bug: 1105569
Change-Id: I219a89c559b06106b75921ba678d2ab9eb69ea06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298168
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788354}