remove _conf.contains check in pre_vote && elect_self

It is necessary to allow nodes that do not exist in the configuration list to be elected as leaders.

Author: walterzhao

src/braft/node.cpp

@@ -1622,11 +1622,6 @@ void NodeImpl::pre_vote(std::unique_lock<raft_mutex_t>* lck, bool triggered) {
                         " configuration is possibly out of date";
         return;
     }
-    if (!_conf.contains(_server_id)) {
-        LOG(WARNING) << "node " << _group_id << ':' << _server_id
-                     << " can't do pre_vote as it is not in " << _conf.conf;
-        return;
-    }
 
     int64_t old_term = _current_term;
     // get last_log_id outof node mutex
@@ -1681,11 +1676,6 @@ void NodeImpl::elect_self(std::unique_lock<raft_mutex_t>* lck,
                           bool old_leader_stepped_down) {
     LOG(INFO) << "node " << _group_id << ":" << _server_id
               << " term " << _current_term << " start vote and grant vote self";
-    if (!_conf.contains(_server_id)) {
-        LOG(WARNING) << "node " << _group_id << ':' << _server_id
-                     << " can't do elect_self as it is not in " << _conf.conf;
-        return;
-    }
     // cancel follower election timer
     if (_state == STATE_FOLLOWER) {
         BRAFT_VLOG << "node " << _group_id << ":" << _server_id