Hide unused header in codemirror screen

This header object isn't populated, but takes up vertical height.
Its hidden in the normal change view to save space.

Change-Id: Ifc87697afe0ae4fc16d48baa08f1f6c84a8352dd

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CodeMirrorDemo.java

@@ -60,6 +60,7 @@ public class CodeMirrorDemo extends Screen {
   @Override
   protected void onInitUI() {
     super.onInitUI();
+    setHeaderVisible(false);
     add(diffTable = new DiffTable());
   }