Revert no_backout workaround when Mercurial identification of backouts is fixed #5490
Description
When https://bugzilla.mozilla.org/show_bug.cgi?id=2001226 is fixed, we can revert a7b0ec8.
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When https://bugzilla.mozilla.org/show_bug.cgi?id=2001226 is fixed, we can revert a7b0ec8.