Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

src/economy.cpp

@@ -292,10 +292,8 @@ void ChangeOwnershipOfCompanyItems(Owner old_owner, Owner new_owner)
 	 * the client. This is needed as it needs to know whether "you" really
 	 * are the current local company. */
 	Backup<CompanyByte> cur_company(_current_company, old_owner, FILE_LINE);
-#ifdef ENABLE_NETWORK
 	/* In all cases, make spectators of clients connected to that company */
 	if (_networking) NetworkClientsToSpectators(old_owner);
-#endif /* ENABLE_NETWORK */
 	if (old_owner == _local_company) {
 		/* Single player cheated to AI company.
 		 * There are no spectators in single player, so we must pick some other company. */