Dequarantine Burndown chart spec

What does this MR do?

Dequarantines Burndown chart spec. Spec was fixed in this MR !29719 (merged).

Passed on Staging:

• https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/1139898#L418
• https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/1139672

Passed on Master:

• https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/jobs/529837816
• https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/jobs/529602452

Original issue #214686 (closed) was closed already.

Does this MR meet the acceptance criteria?

Conformity

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