Simplify the `passing_build` task namespace

• Removes the ce task, since we don't trigger builds for it.
• Removes the all task, since the only remaining task is ee.
• Removes the passing_build helper since it's only used in one place.
• Removes the append_ee? helper, since passing_build:ee should receive a concrete ref such as 12-7-stable-ee.

Also removes the [ce|ee]-green-master-build-trigger jobs, as they've been disabled for the last 6 months.