Update guidance for GitHub related specs in Distribution pipelines

Summary

GITHUB_ACCESS_TOKEN is being mentioned as required env variable in /doc/development/gitlab-qa/index.md, however it's been deprecated in favour of QA_GITHUB_ACCESS_TOKEN which is stored in Quality vault.

Steps:

• Review if GH tests are needed for new GitLab linux packages testing
  • If not needed:
    • remove mention of GITHUB_ACCESS_TOKEN from dev guide
    • add github tag to missing GH related specs (example)
    • ensure --tag ~github is added to QA tags

Context

gitlab-org/omnibus-gitlab!7099 (comment 1542808289)