[#1912] remove all mentions of downloading postgres ourselves

iRODS 4.0+ will use the system-provided packaged PostgreSQL as often
as possible (the same is true for MySQL and Oracle).  It will no
longer attempt to download, install, and manage its own instance of
a database.
[#1858] remove mention of modules from configuration step

Modules are no longer the way to extend iRODS 4.0+.  There is
no reason for the configure scripts to take into consideration
the modules directory or anything in it.

Removing the (now) misleading output and comments for clarity.