Kubernetes Agent API is enabled, simplify instructions

What does this Merge Request do?

Removes manual feature flag toggling as it's no longer needed because the needed API is enabled by default gitlab!41045 (merged).

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.