Remove unreachable clusters controller actions

What does this MR do and why?

Removes unused cluster creation actions and related code from ClustersController.

Some of these changes were meant to happen as part of #357309 (closed), namely the removal of the create_aws and authorize_aws_role actions/routes - these need to be removed because it is technically still possible (though incredibly unlikely) for users to create EKS clusters without the UI by mangling requests.

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