Revert "Quarantine GitHub OAuth test"

What does this MR do?

De-quarantines GitHub OAuth tests. See: https://gitlab.com/gitlab-org/quality/nightly/issues/83#note_145843823

What are the relevant issue numbers?

Closes: https://gitlab.com/gitlab-org/quality/nightly/issues/83

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer