Rollback #968

If you want to prevent submit, use this instead:

```
<Form @submit.native.prevent>
```

src/components/form/form.vue

@@ -1,5 +1,5 @@
 <template>
-    <form :class="classes" @submit="formSubmit"><slot></slot></form>
+    <form :class="classes"><slot></slot></form>
 </template>
 <script>
     // https://github.com/ElemeFE/element/blob/dev/packages/form/src/form.vue
@@ -75,9 +75,6 @@
                 if (!field) { throw new Error('[iView warn]: must call validateField with valid prop string!'); }
 
                 field.validate('', cb);
-            },
-            formSubmit (event) {
-                event.preventDefault();
             }
         },
         watch: {