Eliminates erroneous NULL check that causes crashes in valid SAV

Resubmit of https://github.com/the3dfxdude/7kaa/pull/18 with new comments under a separate branch since it fixes a different bug.

From the code comments: 
This check seems to be deprecated. The code already handles NULL townPtrs with linkout() in TownArray::read_file(). Leaving this in causes crashes when SAV files have deleted towns that were validly written by TownArray::write_file() without DEBUG turned on.