[ng] texi: drop support for '.txi' and '.texinfo' suffixes

[ng] texi: drop support for '.txi' and '.texinfo' suffixes

The former was only needed to cater to 8+3 file systems (for which
we have dropped support anyway), and the latter was hardly used in
practice, it's extra "eye-candy" not worth complicating the code.
We now only accept and handle files with a '.texi' suffix.

* NG-NEWS: Update.
* automake.in (handle_texinfo_helper): Drop support for '.txi'
and '.texinfo' suffixes.
(scan_texinfo_files): Likewise.
* lib/am/texibuild.am: Simplify accordingly, assuming '.texi'
as the only valid suffix for Texinfo input files.
* t/txinfo-rules-once.sh: Resurrect this test, and enhance it
a little, also merging in the suitably adapted contents of ...
* t/txinfo9.sh: ... this test, now deleted.
* t/txinfo6.sh: Remove as obsolete.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>