Ajax: Simplify jQuery.ajaxSettings.xhr

Previously, jQuery.ajaxSettings.xhr, contents were wrapped in a try-catch
as we defined jQuery.support.ajax & jQuery.support.cors executed during the
jQuery load and we didn't want to crash if IE had native XHR disabled (which
is possible). While jQuery hasn't supported the ActiveX-based XHR since 2.0,
jQuery with XHR disabled could still be used for its other features in such
a crippled browser.

Since gh-4347, jQuery.support.ajax & jQuery.support.cors no longer exist, so
we don't need the try-catch anymore.

Fixes gh-1967
Closes gh-4467
Ref gh-4347

src/ajax/xhr.js

@@ -6,9 +6,7 @@ define( [
 "use strict";
 
 jQuery.ajaxSettings.xhr = function() {
-	try {
-		return new window.XMLHttpRequest();
-	} catch ( e ) {}
+	return new window.XMLHttpRequest();
 };
 
 var xhrSuccessStatus = {