[rpc] Remove auth cookie on shutdown

Accidentally removed in 40b556d374

Github-Pull: #10139
Rebased-From: 4b87973c32

src/rpc/server.cpp

@@ -331,6 +331,7 @@ void StopRPC()
 {
     LogPrint("rpc", "Stopping RPC\n");
     deadlineTimers.clear();
+    DeleteAuthCookie();
     g_rpcSignals.Stopped();
 }