fix($compile): remove redundant attr.specified

attr.specified is deprecated in Gecko 7.0 and creates unwanted
warnings on FireFox.  It seems that modern browsers, but possibly not
IE, only provide specified attributes when calling element.getAttributes()
and so removing this is unlikely to break applications.
See https://developer.mozilla.org/en-US/docs/Web/API/Attr and
http://msdn.microsoft.com/en-us/library/ie/ms534637(v=vs.85).aspx

Closes #2160