ValidateEndpoint: remove obsolete usage of StringBuilder

 src/main/java/org/keycloak/protocol/cas/endpoints/ValidateEndpoint.java

@@ -46,10 +46,8 @@
    }

    protected Response successResponse() {
        StringBuilder sb = new StringBuilder(RESPONSE_OK);
        sb.append(clientSession.getUserSession().getUser().getUsername());
        sb.append("\n");
        return Response.ok(sb.toString()).type(MediaType.TEXT_PLAIN).build();
        String response = RESPONSE_OK + clientSession.getUserSession().getUser().getUsername() + "\n";
        return Response.ok(response).type(MediaType.TEXT_PLAIN).build();
    }

    protected Response errorResponse(CASValidationException e) {