Remove standalone="yes" from xml output

This is complicated: a validating parser needs to know how to handle
whitespace (as ignorable markup or as a whitespace node). The default is
to treat it as a whitespace node, and the standalone="yes" tells it that
the document can be handled as such, since there is no internal doctype
definition that says otherwise. But then, when we try to validate
against our DTD, the parser sees that some elements are defined with
element-only content, which conflicts with the standalone default.

References:
http://www.w3.org/TR/REC-xml/#sec-rmd
http://bytes.com/topic/net/answers/553902-standalone-yes
Fix detection of CVE-2011-2523

http://seclists.org/nmap-dev/2014/q4/130