Drop out of date comment from site_cubicweb

We can use db-create/db-init directly now, no need for manual "add
postgis" steps.

site_cubicweb.py

-
-# USE
-# cubicweb-ctl db-create <instance>
-# WARNING !!! DO NOT initialize the database
-# Add postgis, e.g.:
-# psql <database> -f /usr/share/postgresql/9.1/contrib/postgis-1.5/postgis.sql
-# psql <database> -f /usr/share/postgresql/9.1/contrib/postgis-1.5/spatial_ref_sys.sql
-# cubicweb-ctl db-init <instance>
-
 from logilab.database import FunctionDescr, AggrFunctionDescr
 from logilab.database import get_db_helper
 from logilab.database.sqlgen import SQLExpression