chore(auth): Move deleteUser to use case

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:
Moves the delete user functionality to a use case and adds unit tests covering this functionality.

How did you test these changes?
Unit tests
Test app

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

The base branch was changed.

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 41.91%. Comparing base (7f874ed) to head (da0256d).
Report is 60 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3015      +/-   ##
==========================================
- Coverage   44.01%   41.91%   -2.11%     
==========================================
  Files         926      992      +66     
  Lines       30485    31495    +1010     
  Branches     4351     4636     +285     
==========================================
- Hits        13419    13202     -217     
- Misses      15601    16855    +1254     
+ Partials     1465     1438      -27     

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