Drop unnecessary postinst script

We don't actually use anything inside it and the printed "TODO"
isn't really something which should be kept :)

From: Michael Prokop <mprokop@sipwise.com>

debian/postinst

@@ -1,25 +0,0 @@
-#!/bin/sh
-# postinst script for ngcp-db-schema
-
-set -e
-
-case "$1" in
-    configure)
-      echo TODO
-    ;;
-
-    abort-upgrade|abort-remove|abort-deconfigure)
-    ;;
-
-    *)
-        echo "postinst called with unknown argument \`$1'" >&2
-        exit 1
-    ;;
-esac
-
-# dh_installdeb will replace this with shell code automatically
-# generated by other debhelper scripts.
-
-#DEBHELPER#
-
-exit 0