removing the HTML Xref Mismatch node

[Patrice Dumas]

Hello,

While we are on the subject of HTML Xref, I think that the "HTML Xref
Mismatch" node should be removed

 
https://www.gnu.org/software/texinfo/manual/texinfo/html_node/HTML-Xref-Mismatch.html

The information is still true, but I do not think that the solution
proposed is useful in practice as:
1) in general both split and non-split manuals are generated
2) with htmlxref.cnf the nature of the target manual is known

I suggest removing the node from the manual, and put the code example
somewhere else (maybe in tp/TODO).

Opinions?

-- 
Pat