remove unused env_variables region tag

[LukeSchlangen]

Description

Fixes #

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Checklist

• [ ] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [ ] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [ ] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] 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 sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners 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[bot]]

Here is the summary of changes.

You are about to delete 1 region tag.

- [appengine/standard/mailjet/app.yaml:23](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/a457ca68fcf25af3d4729197cd046de2e13db7e4/appengine/standard/mailjet/app.yaml#L23), tag `env_variables`

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