Clean up refresh fix for cancel automatic merge

In !26232 (merged), we fixed the merge request widget not updating properly when clicking the Cancel automatic merge button.

This fix was a bit of hack: it would force the frontend to refresh the merge request widget instead of relying on the output of the /cancel_auto_merge response. This response returned incomplete information when we split the widget information in !15045 (merged) to return cached and uncached data.

In !29757 (merged), we fixed the /cancel_auto_merge response to return all the relevant data, so we can remove this workaround.