Review unused declarations within `io.spine.server` package

[yevhenii-nadtochii]

During implementation of #1436 some unused declarations were revealed.

To address this issue, one should review the "unused" pieces. And either remove them, or suppress the warning with a corresponding comment.

In io.spine.server.model.DuplicateHandlerMethodError on line 54:

    DuplicateHandlerMethodError(Collection<? extends HandlerMethod<?, ?, ?, ?>> handlers) {
        super(format("Handler methods %s are clashing.%n" +
                             "Only one of them should handle this message type.",
                     METHOD_JOINER.join(handlers)));
    }

In io.spine.server.commandbus.given.SingleTenantCommandBusTestEnv on line 131:

        public List<Message> handledCommands() {
            return unmodifiableList(handledCommands);
        }