ApiFormatXml: remove broken xmldoublequote param

r55641 was a refactoring of ApiFormatXml::recXmlPrint() that added
a $doublequote parameter, yet the parameter was not actually passed
along in recursive calls, rendering the formatter's xmldoublequote
parameter useless.

XPath support was the sole reason this parameter was added. It was
argued that because in the XPath 1.0 grammar, a Literal is:

    '"' [^"]* '"' | "'" [^']* "'"

it is not possible to match attribute values that contain both an
apostrophe and a double quote.

However, conforming XPath implementations include a concat() function
that can be used to work around this limitation. Furthermore, XPath 2.0
allows escaping a double-quoted " as "" (and a single-quoted ' as '').

Because there was no Bugzilla report (other than mine) filed in the
last 3-1/2 years, and working around the XPath flaw is possible, I
think it's safe to remove the parameter. The worst that could happen
is that a not-very-robust API client chokes on the "Unrecognized
parameter" warning.

Bug: 46626
Change-Id: I9a4d8c13b78ffd2634d03c8c8b4bbeff69f4bc08