gone_in: make it __printflike()

Provide flexibility to use format strings for gone_in(). As a side
effect, this removes hardcoded string "Obsolete code will be removed
soon:" from the message, so now it is obligation of the deprecated code to
provide a meaningful message. This required a small adoption of the
existing users: midi, drm2 and le(4). Note that gone_in() is not a public
KPI as it has no sense to use it outside of the FreeBSD tree.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50783