jQuery.ajax: getAllResponseHeaders hack is no longer needed

This was fixed in FF21 a while ago as well as in webkit and blink:
https://bugs.webkit.org/show_bug.cgi?id=41210
https://code.google.com/p/chromium/issues/detail?id=87338
https://bugs.webkit.org/show_bug.cgi?id=76419
https://bugzilla.mozilla.org/show_bug.cgi?id=608735

confirmed using  http://jsfiddle.net/xeBub/7/ in browserstack