SEC-584: Remove use of default SessionRegistryImpl.

core/src/main/java/org/springframework/security/concurrent/ConcurrentSessionControllerImpl.java

@@ -40,7 +40,7 @@ public class ConcurrentSessionControllerImpl implements ConcurrentSessionControl
     //~ Instance fields ================================================================================================
 
     protected MessageSourceAccessor messages = SpringSecurityMessageSource.getAccessor();
-    private SessionRegistry sessionRegistry = new SessionRegistryImpl();
+    private SessionRegistry sessionRegistry;
     private boolean exceptionIfMaximumExceeded = false;
     private int maximumSessions = 1;