Remove instance-level cluster specific methods from DeploymentPlatform
In !12740 (comment 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.