Drop support for XeTeX + TeX fonts + inputenc utf8.

Drop support for XeTeX + TeX fonts + inputenc utf8.

LyX follows LaTeX in dropping support for this combination
(it only worked by tricking "inputenc.sty").
There is no known case where this combination is required or helpfull.

For power users with special needs, XeTeX + TeX fonts is still
available after setting the input encoding to "ascii" or "utf8-plain".

See also #10600.