Core Update 166: Drop orphaned convert-ovpn script

Core Update 166: Drop orphaned convert-ovpn script

This script only appeared in conjunction with Core Update 75, released
January 2014. Although it is still being executed while restoring a
backup, it would only be effective if anyone tried to restore a backup
created before C75.

I don't think there is a realistic need to carry this script along any
further. In doubt, it might be better to start from scratch again rather
than trying to restore an 8 year old backup, expecting everything to be
peachy and vanilla with it.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>