Revert "Add iterations cadences to Gitlab Migration"

What does this MR do and why?

Reverts !96570 (merged) as it was causing qa/specs/features/ee/api/1_manage/gitlab_migration_group_spec.rb:133 to fail as described in #367568 (closed)

Check-list

• General code guidelines check-list
  • Code review guidelines
  • Style guides
• Quarantine test check-list
  • Follow the Quarantining Tests guide.
  • Confirm the test has a quarantine: tag with the specified quarantine type.
  • Note if the test should be quarantined for a specific environment.
  • (Optionally) In case of an emergency (e.g. blocked deployments), consider adding labels to pick into auto-deploy (Pick into auto-deploy priority1 severity1).
• Dequarantine test check-list
  • Follow the Dequarantining Tests guide.
  • Confirm the test consistently passes on the target GitLab environment(s).
• 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.