fix: do not call unregister_rpc_handler when exiting progress

[zhangyifan27]

When exiting progress dsn::task::get_current_node() is nullptr, so we shouldn't call dsn_rpc_unregiser_handler. Also, when the rpc_engine destructed, all handlers have been cleared, this is no need to call dsn_rpc_unregiser_handler.

rpc_server_dispatcher::~rpc_server_dispatcher()
{
    for (auto &h : _vhandlers) {
        delete h;
    }
    _vhandlers.clear();
    _handlers.clear();
    dassert(_handlers.size() == 0,
            "please make sure all rpc handlers are unregistered at this point");
}