tests: drop requirement 'makeinfo-html'; 'makeinfo' is enough

tests: drop requirement 'makeinfo-html'; 'makeinfo' is enough

* defs: Since we now require Texinfo 4.9 or later in the Automake
generated Makefiles, and since the '--html' option of makeinfo, as
well as its capability to generate HTML output, has been supported
since Texinfo 4.0 (according to the Texinfo NEWS file), the
requirement 'makeinfo-html' is obsolete -- the simpler requirement
'makeinfo' is enough.
* t/instdir-texi.sh ($required): Adjust by using simply 'makeinfo'
instead of 'makeinfo-html'.
* t/silent8.sh: Likewise.
* t/txinfo21.sh: Likewise.

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