Simplify feature flag setup in e2e test pipelines

What does this MR do and why?

• With E2E: Move feature flag setup to rspec hooks (!135809 - merged) merged environment variable QA_FEATURE_FLAGS can be used directly without the need to construct cli command thus simplifying the setup
• EXTRA_GITLAB_QA_OPTS was added to allow super sidebar tests run alongside the normal pipeline and should not be relevant anymore

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.