Ajax: simplify one ajax call and add explanatory comment

* Remove "async = true" from script transport since it was needed
  for FF < 4 and old Opera which we do not support anymore

* Add comment to "evalUrl" method on why "type" field should be explicit

Ref 0ac28ed293681cb8f2e9fdd11efa0021da039c84

src/ajax/script.js

@@ -46,8 +46,6 @@ jQuery.ajaxTransport( "script", function(s) {
 
 				script = document.createElement("script");
 
-				script.async = true;
-
 				if ( s.scriptCharset ) {
 					script.charset = s.scriptCharset;
 				}

src/manipulation/_evalUrl.js

@@ -8,6 +8,8 @@ jQuery._evalUrl = function( url ) {
 		type: "GET",
 		dataType: "script",
 		cache: true,
+
+		// Make this explicit, since user can override this through ajaxSetup (#11264)
 		async: false,
 		global: false,
 		"throws": true