Dequarantine modify_design_content_spec test

What does this MR do?

Dequarantines modify_design_content_spec test that is passing reliably now. The failure that prompted quarantining was recorded as consistent from the 27th of May (multiple failures on different environments per day) - see gitlab-org/quality/testcases#1760 (comment 586005656). However, it stopped abruptly around the 13th of July and there have been no more failures recorded in the past month, so it is highly likely a change that fixed it was implemented around that time.

E2E Test Failure issue(s)

#331978 (closed)

Check-list

• General code guidelines check-list
  • Code review guidelines
  • Style guides
• Dequarantine test check-list
  • Follow the Dequarantining Tests guide.
  • Confirm the test consistently passes on the target GitLab environment(s).
    • [-] (Optionally) Trigger a manual GitLab-QA pipeline against a specific GitLab environment using the RELEASE variable from the package-and-qa job of the current merge request.
• To ensure a faster turnaround, ask in the #quality Slack channel for someone to review and merge the merge request, rather than assigning it directly.