[ng] texinfo: don't support split info files anymore

[ng] texinfo: don't support split info files anymore

They were once required for performance or memory-saving reasons; but
such considerations, while certainly relevant in the past, will,
thanks to today's powerful machines, get less and less important in
the future, shrinking to irrelevance soon enough ("soon" for what
concerns the Automake-NG timetable, at least).

So, since the need to cater to split '.info' output files adds some
annoying complexity in some places of the Automake-NG codebase, we
simply stop supporting them.  This change should actually be almost
transparent to the users.

References:
<http://lists.gnu.org/archive/html/automake-ng/2012-08/msg00116.html>
<http://lists.gnu.org/archive/html/texinfo-devel/2012-08/msg00015.html>

* lib/am/texinfos.mk (.am/install-info, uninstall-info-am,
dist-info, am.clean.maint.f): Simplify moderately.
* lib/am/texibuild.mk (am.texi.build.info): Simplify greatly.
* t/txinfo20.sh: Adjust.

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