Remove user after test

What does this MR do?

Updates a QA test to delete a user that it creates.

See also https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/754

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content