Don't print DockerHub helper image warning when custom image is defined

What does this MR do?

Removes the Pulling GitLab Runner helper image from Docker Hub... warning message from output when a custom helper image is defined for the Kubernetes executor.

Why was this MR needed?

We're migrating to use registry.gitlab.com as the main source of the helper images, when they are pulled externally. But as this may be a breaking change for some environments, we've currently added this behavior hidden behind a feature flag and started printing a warning about our migration plans, when the flag is not used.

The user may however define a custom helper image to use, and in that case it doesn't make any sense to print the warning.

In Docker executor this is respected, and the warning is used only when the flag is not set and when a default image is set for use. If user defines a custom helper image in config.toml, warning is not printed at all.

This behavior is different in case of the Kubernetes executor, where the warning is printed always if the feature flag is not set. No matter if a custom, user defined helper image is in fact used or not.

This MR fixes this problem and removes the confusion that it creates.

What's the best way to test this MR?

What are the relevant issue numbers?

Fixes #27566 (closed)