Change d/additions/debian-start to no-op

The script was being run during upgrades in various situations. Since it runs
mysql_upgrade, which is also run by d/postinst in 5.7, it was causing various
errors. All its functionality is also covered by d/postinst, so it's no longer
needed.
* Changed debian-start to just exit 0
* Removed debian-start.inc.sh, which was only used by debian-start
LP: #1577712