[rpc] Remove auth cookie on shutdown

Accidentally removed in 40b556d374

src/rpc/server.cpp

@@ -322,6 +322,7 @@ void StopRPC()
 {
     LogPrint(BCLog::RPC, "Stopping RPC\n");
     deadlineTimers.clear();
+    DeleteAuthCookie();
     g_rpcSignals.Stopped();
 }