GUACAMOLE-364: revert unnecessary code simplification

This change isn't necessary to the stated issue. The expression
here is unnecessarily complex but that should be addressed in
another issue.

guacamole/src/main/java/org/apache/guacamole/rest/auth/AuthenticationService.java

@@ -295,7 +295,7 @@ public class AuthenticationService {
             }
 
             // Otherwise, attempt authentication as a new user
-            AuthenticatedUser authenticatedUser = authenticateUser(credentials);
+            AuthenticatedUser authenticatedUser = AuthenticationService.this.authenticateUser(credentials);
             notifyAuthenticationSuccessListeners(authenticatedUser, null);
 
             if (logger.isInfoEnabled())