postgres-init-db: The zulip user doesn't need CREATE DB on enterprise.

Also clean up the series of similar blocks.

(imported from commit fdb468bd6be3077bdbf2d72289064397ea04f27a)

scripts/setup/postgres-init-db

@@ -2,16 +2,9 @@
 
 su postgres -c psql <<EOF
 CREATE USER zulip;
-ALTER USER zulip CREATEDB;
 ALTER ROLE zulip SET search_path TO zulip,public;
-EOF
-
-su postgres -c psql <<EOF
 DROP DATABASE IF EXISTS zulip;
 CREATE DATABASE zulip OWNER=zulip;
-EOF
-
-su postgres -c psql <<EOF
 DROP SCHEMA public CASCADE;
 EOF