Remove instance-level cluster specific methods from DeploymentPlatform
In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12740#note_171261371 it was noticed that DeploymentPlatform#find_instance_cluster_platform_kubernetes
was failing despite there being specific specs to check it.
It turns out DeploymentPlatform#find_group_cluster_platform_kubernetes
is already returning instance level clusters correctly and so find_instance_cluster_platform_kubernetes
is never called in the specs testing it.
We should remove all the instance-level cluster methods from DeploymentPlatform and make sure its clear the existing methods are applicable to the entire cluster hierarchy not just groups.
This was likely caused by modifying DeploymentPlatform to add instance level clusters before modifying the more low level classes then having no specs break.