net: only delete CConnman if it's been created

In the case of (for example) an already-running bitcoind, the shutdown sequence
begins before CConnman has been created, leading to a null-pointer dereference
when g_connman->Stop() is called.

Instead, Just let the CConnman dtor take care of stopping.

src/init.cpp

@@ -200,7 +200,6 @@ void Shutdown()
         pwalletMain->Flush(false);
 #endif
     MapPort(false);
-    g_connman->Stop();
     g_connman.reset();
 
     StopTorControl();