Remove deprecated PostgreSQL database driver

This was deprecated for removal in Pike. It's probably time to drop it.
Note that the 'postgresql-devel'/'postgresql-server-dev-all' packages
are retained since some packages still include 'psycopg2' in their
general requirements.

Change-Id: I51e8354e99972757253ce259e6c03c91da24398c
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>
diff --git a/functions b/functions
index f33fd25..8eeb032 100644
--- a/functions
+++ b/functions
@@ -400,7 +400,8 @@
 # initialized yet, just save the configuration selection and call back later
 # to validate it.
 #
-# ``$1`` - the name of the database backend to use (mysql, postgresql, ...)
+# ``$1`` - the name of the database backend to use (only mysql is currently
+# supported)
 function use_database {
     if [[ -z "$DATABASE_BACKENDS" ]]; then
         # No backends registered means this is likely called from ``localrc``