Remove a very outdated Gozag shops comment

It appears since c8711a1 it hasn't been true that serial_shops is used
by Gozag code. Prior to that it was the case that Gozag would only call
merchants to branches that already allowed random shops, so the comment
is long redundant now.

crawl-ref/source/dat/des/builder/shops.des

@@ -58,8 +58,6 @@ function ks_shop_vault(e, glyphs)
 end
 }}
 
-# XXX: Don't change the name of this vault!
-# Gozag code uses it to find out where shops can normally place.
 NAME: serial_shops
 TAGS: nolayout_encompass allow_dup extra luniq
 DEPTH: D, Depths, Orc, Elf, Shoals, Snake, Vaults