Remove prefix/local support in elements and attributes (#1413)

These parts of element and attribute name are easy to extract.
Now we can easily replace attrs with xast attributes object.

plugins/convertEllipseToCircle.js

@@ -32,8 +32,6 @@ exports.fn = function (item) {
       item.addAttr({
         name: 'r',
         value: radius,
-        prefix: '',
-        local: 'r',
       });
     }
   }