Revert "Import the proper HandlerRegistration (#8420)"

This reverts commit d6595af90d.

src/com/vaadin/terminal/gwt/client/ServerConnector.java

@@ -6,7 +6,7 @@ package com.vaadin.terminal.gwt.client;
 import java.util.Collection;
 
 import com.google.gwt.event.shared.GwtEvent;
-import com.google.gwt.event.shared.HandlerRegistration;
+import com.google.web.bindery.event.shared.HandlerRegistration;
 import com.vaadin.terminal.gwt.client.communication.ClientRpc;
 import com.vaadin.terminal.gwt.client.communication.SharedState;
 import com.vaadin.terminal.gwt.client.communication.StateChangeEvent.StateChangeHandler;