Remove custom CNG-mirror fork for test-on-cng pipeline

What does this MR do and why?

Now that all improvements have landed in main CNG repo, remove custom quality fork for downstream builds.

Related to: Integrate all implemented changes in Quality CN... (gitlab-org/quality/quality-engineering/team-tasks#2841 - closed)
Implements: Update CI setup for test-on-cng in gitlab project (gitlab-org/quality/quality-engineering/team-tasks#2996 - closed)

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