psql: remove old, inaccurate, duplicate, and unused install procedure

Turns out that we don't use the `postgresql` module at all right now,
and `dip::database` has been removed. We have an Installation section now.

howto/postgresql.md

@@ -20,20 +20,6 @@ and so on.
 
 # How-to
 
-## Deployment
-
-A PostgreSQL server should be deployed with puppet, using the
-`postgresql` module. Make sure you also include the
-`postgres::backup_source` class so that it's registered with the
-backup server, see the `dip::database` class for an example on how to
-deploy a server.
-
-See the [howto/puppet](howto/puppet) documentation for more information on how to deploy
-Puppet things.
-
-TODO: turn this in a Tutorial by making it more straightforward to
-deploy a cluster, for example just by including a single class.
-
 ## Find what is taking up space
 
 This will report size and count information for all "relations", which