MenuItem: Remove unused default constructor

The default constructor was only used by GWT, which has been removed.

Change-Id: Ib491a55086e99f19bd9a98e1448c66ef0da55558
diff --git a/java/com/google/gerrit/extensions/client/MenuItem.java b/java/com/google/gerrit/extensions/client/MenuItem.java
index 8375bba..0c7dd88 100644
--- a/java/com/google/gerrit/extensions/client/MenuItem.java
+++ b/java/com/google/gerrit/extensions/client/MenuItem.java
@@ -22,11 +22,6 @@
   public final String target;
   public final String id;
 
-  // Needed for GWT
-  public MenuItem() {
-    this(null, null, null, null);
-  }
-
   public MenuItem(String name, String url) {
     this(name, url, "_blank");
   }