Remove confusing PostgreSQL data source

I spent several hours trying to get this working with a MySQL data source by editing this file, and wondering why my environment variables, e.g., `OPENSHIFT_MYSQL_DB_HOST`, were evaluating to empty strings instead of to the correct values.

It turns out that environment variables are just never evaluated in this file at all. The actual data sources are declared in `jbossas/standalone/configuration/standalone.xml` on the Gear (where environment variables *are* evaluated), so there is no need to define the PostgreSQL or MySQL data sources in the `-ds.xml` file.

I propose deleting this part of the config file to avoid someone else getting as confused as me in future :)