chore(cloudrun): remove older region tags

[hivanalejandro]

Description

Remove unused "cloudrun_pubsub_dockerfile" region tag

Fixes b/393605656

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• run/pubsub/Dockerfile:16, tag cloudrun_pubsub_dockerfile

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[iennae]

this is the snippet where all languages need to be updated and the doc needs to be updated to use the new one before any of them can be deleted.