msxml3/element: Implement removeAttributeNode function.
Implement the function IXMLDOMElement_removeAttributeNode. Essentially
pass the hard work to IXMLDOMNamedNodeMap_removeNamedItem.
Use the fact, that attribute names are unique in elements.
One case that isn't checked is wheter or not the value of the provided attribute match with the one stored in the dom-element.
This MR implements according to the tests introduced in !8928 (merged)
Edited by Reinhold Gschweicher