infra: Remove outdated "master" ref in CI config. This should let the CI console start running changes again. Bug: angleproject:6685 Change-Id: I2d172bd9a90b9772efeeaa9cadf0bc865b917e16 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3276516 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>