Revert "Made UI focusable again (#11129)"

This reverts commit 3cf6913c9a

Change-Id: Ief84593763402f99f41a6a447db951196c8f4a91

server/src/com/vaadin/ui/UI.java

@@ -78,7 +78,7 @@ import com.vaadin.util.CurrentInstance;
  * @since 7.0
  */
 public abstract class UI extends AbstractSingleComponentContainer implements
-        Action.Container, Action.Notifier, LegacyComponent, Focusable {
+        Action.Container, Action.Notifier, LegacyComponent {
 
     /**
      * The application to which this UI belongs