fix: remove backend core directory

[ludtkemorgan]

This PR addresses #(insert-number-here)

• [ ] Addresses the issue in full
• [ ] Addresses only certain aspects of the issue

Description

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How Can This Be Tested/Reviewed?

Provide instructions so we can review, including any needed configuration, and the test cases that need to be QAd.

Author Checklist:

• [ ] Added QA notes to the issue with applicable URLs
• [ ] Reviewed in a desktop view
• [ ] Reviewed in a mobile view
• [ ] Reviewed considering accessibility
• [ ] Added tests covering the changes
• [ ] Made corresponding changes to the documentation
• [ ] Ran yarn generate:client and/or created a migration when required

Review Process:

• Read and understand the issue
• Ensure the author has added QA notes
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• Pull the changes down locally and test that the acceptance criteria is met
• Either (1) explicitly ask a clarifying question, (2) request changes, or (3) approve the PR, even if there are very small remaining changes, if you don't need to re-review after the updates

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