Remove :create_eks_clusters feature flag

What does this MR do?

Removes create_eks_clusters feature flag.

Screenshots

Cluster selection screen

Authorise AWS role

Configure EKS cluster

EKS cluster creating

EKS cluster created + Helm installed

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Configure GKE cluster

GKE cluster creating

GKE cluster created + Helm installed

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

#22392 (closed)