Remove GitHub OAuth e2e spec

What does this MR do?

See the reasoning behind removing this test here: #196517 (comment 277828505)

✅ - Should be merged after gitlab-qa!416 (merged) is merged and a new gem is published

Closes #196517 (closed)

Screenshots

Does this MR meet the acceptance criteria?

test

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content