fix regression in the listBackupSchedules API

Author: bernardodemarco

api/src/main/java/org/apache/cloudstack/api/command/user/backup/ListBackupScheduleCmd.java

@@ -59,7 +59,6 @@ public class ListBackupScheduleCmd extends BaseListProjectAndAccountResourcesCmd
     @Parameter(name = ApiConstants.VIRTUAL_MACHINE_ID,
             type = CommandType.UUID,
             entityType = UserVmResponse.class,
-            required = true,
             description = "ID of the Instance")
     private Long vmId;