E2E test: remove mentions of QA_DEBUG env variable

What does this MR do and why?

QA_DEBUG is no longer used in the codebase, removing

gitlab-org/quality/quality-engineering/team-tasks#3859 (closed)

MR for gitlab-qa: gitlab-qa!1434 (merged)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.