tests/perfmodels/value_nan.c: remove un-needed declaration

tests/perfmodels/value_nan.c

@@ -31,7 +31,6 @@ int _check_number(double val, int checknan)
 {
 	char *tmp = "starpu_XXXXXX";
 	char filename[100];
-	int id;
 
 	strcpy(filename, tmp);