Remove Kubernetes service integration page

What does this MR do?

This MR does

• removes the deprecated KubernetesService page (Project > Integrations > Kubernetes)
• does not remove KubernetesService page (Admin Area > Service Templates > Kubernetes)
• Removes duplicated Kubernetes related logic from KubernetesService

QA manual

• please let's not forget to test the service template to be working also after the cleanup!

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39217 Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41413