Remove docker-registry-mirror

[soutenniza]

We maintain a docker hub mirror to avoid hitting API request limits. Since we're switching to ECR, Concourse will no longer pull images directly from Docker Hub except to build our hardened versions. This means we will probably fall below API usage limits and we can likely remove this.

• Confirm our expected usage will fall below Docker Hub usage limits
• If expected usage is below usage limits, remove the mirror

[dandersonsw]

Since we are switching to using ECR, we shouldn't need the docker-registry-mirror?

[dandersonsw]

I was able to locate the final pipelines using the docker-registry-mirror (these were in staging). Now that those have been updated or paused we can remove the docker-registry-mirror completely.

[dandersonsw]

PR to remove the docker-registry-mirror from being used by concourse: cloud-gov/deploy-concourse#240

[dandersonsw]

Will run the above PR at the start of next year so we don't cause any issues with cert rotation