[ng] texi: drop support for suffix-less info files

[ng] texi: drop support for suffix-less info files

Such support is seldom used, and will get in the way of future, useful
refactorings.  We can still re-introduce it at a later time, after the
dust is settled, if there are users' requests in this direction.

* automake.in (scan_texinfo_file): Error out if a input '.texi' file
specifies an output (in the '@setfilename' directive) that doesn't have
a '.info' extension.
(output_texinfo_build_rules): Simplify, by assuming generated info
files ought to have the '.info' extension.
* lib/am/texibuild.am: Likewise.
* NG-NEWS: Update
* t/txinfo3.sh: Remove as obsolete.
* t/txinfo-unrecognized-extension-2.sh: New test.
* t/primary-prefix-valid-couples.sh: Adjust to avoid spurious failures.
* t/txinfo17.sh: Likewise.
* t/txinfo20.sh: Likewise.
* t/vtexi2.sh: Likewise.
* t/mdate2.sh: Likewise.

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