Delete assignments and evaluations when an evaluator is deleted

[stepchud]

What/Why

When an Evaluator is removed from a Challenge phase, we need to remove the other associations if they exist.
Currently the EvaluatorManagementService is only deleting the ChallengePhaseEvaluator, however it should also delete any EvaluatorSubmissionAssignment and Evaluation records associated with that user, if any exist.
NOTE: there is already a modal that warns the user this will happen, but the backend is not implemented.

Acceptance Criteria

1. Create an evaluator user on a challenge phase
2. Assign them to a submission
3. Complete an evaluation from the user
4. Delete the evaluator from the challenge phase

• The EvaluatorSubmissionAssignment should be deleted from the DB.
• The Evaluation should be deleted from the DB.

Disregard the following if this work does not include any UI changes

• WAVE screenshot
• Automated a11y tests (axe-rspec specs)
• Keyboard accessibility (tab navigation, form interactions)
• Responsiveness at desktop and mobile

Out of Scope