Drop extraneous flash message when rebase error occurs

What does this MR do and why?

When a rebase error occurs, we already display the rebase error in the merge request widget next to the Rebase button. The flash message at the top that says, Something went wrong. Please try again. doesn't add any value and distracts from the main error message in the window. Let's drop it to clean up the interface.

Relates to #346814

Screenshots or screen recordings

Before

After

How to set up and validate locally

1. In a project, enable fast-forward merges in Settings -> General.
2. Create a new file a.txt and a merge request for that.
3. Create a new file b.txt and a merge request for that.
4. Merge one of the merge requests.
5. Set a push rule to ban pushes without a commit message (e.g. ACME).
6. Visit the other merge request and click Rebase.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.