Fix rebase status check throwing flash message

What does this MR do?

Fix rebase status check throwing flash message

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We switched over to Axios in the vue_merge_request_widget bundle but forgot this one ee/ area, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15951

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Not needed as bug was introduced in 10.4 and is being fixed in 10.4
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4473 (closed)