Remove Kubernetes service integration page

We now have first class support for Kubenetes clusters in CI/CD > Kubernetes.

The old page for the Kubernetes service integration in Settings > Integrations has already been deprecated, and it is now partially working for backward compatibility. In an API endpoint exists it should also be removed.

After these checks have been done, we can completely remove this page:

• Existing data in the service integration page has been migrated to the new CI/CD > Kubernetes model
• Service template still works and creates cluster entries in CI/CD > Kubernetes (existing cluster)
• Demos are not relying on this page anymore
• Deprecate docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18837
• Ensure Kubernetes service template is still working after the removal of the integration

Important I2P demo currently uses this service integration, this should only be removed after the demo has been updated to use instance-level cluster instead.

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