Remove private cluster 'feature' from GCP terraform config

[sgibson91]

Context

We needed this to deploy to a project that didn't allow public subnets. That project has been decommissioned and we expect to require public subnets in the future. This feature also involves deploying a NAT which dramatically increases costs. We should remove it.

Task list

Tasks

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1. Remove any code that references enable_private_cluster in GCP terraform config

Definition of Done

• All references to private cluster and related infrastructure in GCP terraform config has been removed