Change: remove the ability to control "max spectators" (#9466)

Soon we will make "join game" join the game as spectator first,
so limiting the amount of spectators makes no sense anymore in
that context. Not sure it ever did make sense.

src/network/network_func.h

@@ -60,7 +60,6 @@ void NetworkClientSendChat(NetworkAction action, DestType type, int dest, const
 bool NetworkClientPreferTeamChat(const NetworkClientInfo *cio);
 bool NetworkCompanyIsPassworded(CompanyID company_id);
 bool NetworkMaxCompaniesReached();
-bool NetworkMaxSpectatorsReached();
 void NetworkPrintClients();
 void NetworkHandlePauseChange(PauseMode prev_mode, PauseMode changed_mode);