Stop publishing helper image to DockerHub

Overview

With !2540 (merged) and !2554 (merged) we started publish the helper image to DockerHub and in #27218 (closed) it pulls the helper image from registry.gitlab.com by default. We should have 1 single source of truth of where our helper image lives for maintainability perspective and reduce the dependencies we have on 3rd party services that we don't have control over.

Proposal

• Remove the publishing of images for Linux and Windows
• Remove the FF_GITLAB_REGISTRY_HELPER_IMAGE feature flag from the code base and have runner always pull the helper image hosted in registry.gitlab.com.