Remove unused functions reported by cppcheck (#10463)

Run unused functions reported by cppcheck

This change removes a few (but not all) unused functions.
Some unused helper functions were not removed due to their complexity and potential of future use.

src/client/client.h

@@ -338,7 +338,6 @@ class Client : public con::PeerHandler, public InventoryManager, public IGameDef
 	u16 getProtoVersion()
 	{ return m_proto_ver; }
 
-	bool connectedToServer();
 	void confirmRegistration();
 	bool m_is_registration_confirmation_state = false;
 	bool m_simple_singleplayer_mode;