[Trivial][RPC] Remove references to old command list-conf
[Trivial][RPC] Remove references to old command list-conf

`startmasternode` `createmasternodebroadcast` error outputs are still
referencing old command `list-conf` (deprecated since version v2.3.1).
Replace with `listmasternodeconf`.
PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet