classes/sanity: remove texi2html from required host utilities

classes/sanity: remove texi2html from required host utilities

texi2html is not actually required to build world of OE-Core anymore,
so we don't need to mandate it. The only difference without it (verified
with buildhistory) is that groff doesn't produce HTML documentation; the
rest of its docs are still produced and packaged and no other packages
are affected.

Part of the work towards [YOCTO #2423].

(From OE-Core rev: aa1c4519f8e549b43a7d8bc51c5342d3409b464b)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>