Codechange: move all NetworkGameInfo related functions to a single file

It currently was a bit scattered over the place. Part of
NetworkGameInfo is also the GRF Identifiers that goes with it.

src/network/core/core.h

@@ -71,8 +71,6 @@ public:
 	 */
 	void Reopen() { this->has_quit = false; }
 
-	void SendGRFIdentifier(Packet *p, const GRFIdentifier *grf);
-	void ReceiveGRFIdentifier(Packet *p, GRFIdentifier *grf);
 	void SendCompanyInformation(Packet *p, const struct Company *c, const struct NetworkCompanyStats *stats, uint max_len = NETWORK_COMPANY_NAME_LENGTH);
 };