Move MathML tree simplification from `mrow.close()` to `parse_latex_math()`.

Move MathML tree simplification from `mrow.close()` to `parse_latex_math()`.

Do not open a new `<mrow>` (only to remove it afterwards)
for commands like ``\sqrt{3 +1}`` (LaTeX macro takes one argument
but `<sqrt>` node accepts a group).

Note: The replacement of `mrow` by a single child node remains
in the `mrow.close()` method, because we don't know in forehand
whether there is only one child.

git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9524 929543f6-e4f2-0310-98a6-ba3bd3dd1d04