fix(radio): select only one radio when w/out ngModel

Closes #5659

ionic/components/radio/test/basic/main.html

@@ -80,7 +80,7 @@
     <code><b>relationship:</b> {{relationship}}</code>
   </div>
 
-  <div radio-group [(ngModel)]="pet" (change)="petChange($event)">
+  <div radio-group (change)="petChange($event)">
     <p>
       <ion-radio (select)="dogSelect($event)"></ion-radio>
       Dogs
@@ -95,10 +95,6 @@
     </p>
   </div>
 
-  <div padding>
-    <code><b>pet:</b> {{pet}}</code>
-  </div>
-
   <ion-list radio-group [(ngModel)]="someValue">
     <ion-item *ngFor="#item of items">
       <ion-label>