bug#57108: Time to retire texinfmt.el

[Stefan Kangas]

I've tested `texinfo-format-buffer' with several manuals from GNU/NonGNU
ELPA packages.

Only one file worked:

    magit/docs/magit-section.texi

In all other cases I tried I got errors

    bnf-mode/bnf-mode.texi

       (error "@comma is not handled by texinfo")

    magit/docs/magit.texi

        (error "Unmatched @end example") ; false positive

    org-roam/doc/org-roam.texi

        (error "@LaTeX is not handled by texinfo")

    slime/doc/slime.texi

        (error "@codequoteundirected is not handled by texinfo")

    swiper/doc/ivy.texi

        (error "@indentedblock is not handled by texinfo")

    use-package/use-package.texi

        (error "Unmatched @end example") ; false positive

The fact that this command has been this broken for so long without
anyone complaining should give some indication of how much used it sees.