Remove invalid broadcast messages before migrating

What does this MR do?

This fixes the migration introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13429 to prevent problems such as the one described in https://gitlab.com/gitlab-org/gitlab-ce/issues/36418 from occurring.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together