rpc: erase empty map entry in removeCommand

After removing the last CRPCCommand pointer for a given name,
erase the now-empty vector from mapCommands. Without this,
listCommands() returns the name of a fully removed command
because it iterates mapCommands keys unconditionally.

For example, when unloading the wallet the RPCs are deregistered, and
this prevents getopenrpcinfo from returning non-existant RPCs.

src/rpc/server.cpp

@@ -264,6 +264,9 @@ bool CRPCTable::removeCommand(const std::string& name, const CRPCCommand* pcmd)
         auto new_end = std::remove(it->second.begin(), it->second.end(), pcmd);
         if (it->second.end() != new_end) {
             it->second.erase(new_end, it->second.end());
+            if (it->second.empty()) {
+                mapCommands.erase(it);
+            }
             return true;
         }
     }