Remove code left over from when clusters were always project-specific
Summary
When clusters were first introduced, they always had a direct association to a project. Now that we have added support for group and instance-level clusters, it is misleading to see code that still implies there will be a single project associated with a cluster.
This causes bugs such as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27812.
Improvements
Make it more obvious that a cluster might not have a single project directly associated, and simplify adding features as we start increasing support for group and instance-level clusters.
Risks
Risks breaking existing project-level clusters
Involved components
app/models/clusters/cluster.rb
app/models/clusters/platforms/kubernetes.rb
Optional: Intended side effects
None