Remove keyup event listener for forms (#184)

Let the browser take care of when to do implicit submission.

lib/proto/attach-form.js

@@ -120,10 +120,4 @@ module.exports = function(el) {
   on(el, "submit", function(event) {
     formAction.call(that, el, event)
   })
-
-  on(el, "keyup", function(event) {
-    if (event.keyCode === 13) {
-      formAction.call(that, el, event)
-    }
-  }.bind(this))
 }