Ajax: simplify jQuery.parseXML method

-2 bytes to the gzipped size and code looks a bit nicer

This wasn't landed to the compat branch though, since size would increase
with this cherry-pick

Closes gh-1856

src/ajax/parseXML.js

@@ -4,15 +4,14 @@ define([
 
 // Cross-browser xml parsing
 jQuery.parseXML = function( data ) {
-	var xml, tmp;
+	var xml;
 	if ( !data || typeof data !== "string" ) {
 		return null;
 	}
 
 	// Support: IE9
 	try {
-		tmp = new DOMParser();
-		xml = tmp.parseFromString( data, "text/xml" );
+		xml = ( new DOMParser() ).parseFromString( data, "text/xml" );
 	} catch ( e ) {
 		xml = undefined;
 	}