Remove bogus comment

The comment was introuced in commit da29f74 for a constant named
CLICK_EVENT_ID, but it was accidentally left in the code when that
constant was removed.

Change-Id: I69697f517585cafb30cc223497bc8db4a919e884

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

@@ -139,7 +139,6 @@ public abstract class UI extends AbstractSingleComponentContainer implements
      */
     protected ActionManager actionManager;
 
-    /** Identifies the click event */
     private ConnectorTracker connectorTracker = new ConnectorTracker(this);
 
     private Page page = new Page(this, getState(false).pageState);