Fix email error copy

[benbowler]

Summary

Addresses issue:

• Explore combining email reporting templates into a single "simple" template #12157

Relevant technical choices

There was some confusion on which email copy to use. This sheet should be used as the single source of truth. This PR fixes the copy.

While I was here I removed a few error keys that were removed through discusson on the error sheet and are no longer used.

I also moved get_body_args to the Content_Map as part of parallel work to decouple the tester plugin so that it doesn't duplicate copy.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

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🎭 Playwright reports for 423974c:

• WordPress 5.2.21
• WordPress latest
• WordPress nightly

📦 Build files for 423974c:

• Download development build
• Download production build

[tofumatt]

Changes here are good, but because this is a release issue but the base branch is develop, I'll cherry-pick these commits into a branch based on main and merge that into main 👍🏻