Remove unnecessary email converter

[bryanwweber]

The homework and exam grades don't have an Email Address column, so the
converter isn't necessary

Where to put new files:

• New files should go into a top-level subfolder, named after the article slug. For example: my-awesome-article

How to merge your changes:

1. Make sure the CI code style tests all pass (+ run the automatic code formatter if necessary).
2. Find an RP Team member on Slack and ask them to review & approve your PR.
3. Once the PR has one positive ("approved") review, GitHub lets you merge the PR.
4. 🎉

From a comment on the article, the "Email Address" converter for the hw_exam_grades.csv file is not needed and potentially confusing.

[somacdivad]

Looks good @bryanwweber 👍

I'll go ahead and merge.