Unset client.conn.playing on disconnect

Since it's used everywhere in the client code to determine the current
player, keeping it set to a dangling value triggers various assertions.

Closes longturn#2195.

client/clinet.cpp

@@ -55,7 +55,8 @@
 static void close_socket_nomessage(struct connection *pc)
 {
   connection_common_close(pc);
-  remove_net_input();
+  pc->playing = nullptr;
+
   popdown_races_dialog();
 
   set_client_state(C_S_DISCONNECTED);

client/gui_main.cpp

@@ -146,13 +146,6 @@ void sound_bell()
  */
 void add_net_input(QIODevice *sock) { king()->add_server_source(sock); }
 
-/**
-   Stop waiting for any server network data.  See add_net_input().
-
-   This function is called if the client disconnects from the server.
- */
-void remove_net_input() {}
-
 /**
    Set one of the unit icons (specified by idx) in the information area
    based on punit.

client/qtg_cxxside.h

@@ -20,7 +20,6 @@ class QTcpSocket;
 void options_extra_init();
 void set_rulesets(int num_rulesets, QStringList rulesets);
 void add_net_input(QIODevice *sock);
-void remove_net_input();
 void real_conn_list_dialog_update(void *unused);
 void sound_bell();