Remove 11.x ABI compat for kernel dump ioctls

This code was marked gone_in(13), so its time has passed.

The only consumer of this interface is dumpon(8). We do not maintain
strict backwards compatibility for this utility because a) it
can't/shouldn't be used from a jail or chroot and b) it is highly
specific interface unique to FreeBSD. The host's (presumably more
up-to-date) copy of dumpon(8) should be used to configure kernel dump
devices.

Reviewed by: markj, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34913