Fix 480937a103708a651/lyxgit, second attempt.

Fix 480937a103708a651/lyxgit, second attempt.

Prevent encoding changes whenever the TeX engine is XeTeX or LuaTeX,
as XeTeX/LuaTeX use only one encoding per document:

* with useNonTeXFonts: "utf8plain",
* with XeTeX and TeX fonts: "ascii" (inputenc fails),
* with LuaTeX and TeX fonts: only one encoding accepted by luainputenc.

+1 no needless encoding switches
+1 runparams.encoding matches the correct encoding at any time
+1 less complicated code.

-1 there may still be problems with CJK (possibly impossible to
   solve for Xe/LuaTeX with TeX fonts).

For LuaTeX & TeX fonts, the complete document uses the encoding
of the global document language.

See also #9740.