databases/mysql56-server: Fix misleading part of pkg-message.
Part of the pkg-message points about .mysql_secret file which
isn't case about new mysql56 release anymore.
Delete no-more-valid part.
(The case still is a thing for 57 and 80)

PR: 225696
Reported by: knezour@weboutsourcing.cz
Sponsored by: Netzkommune GmbH