md(4): remove the unused and unusable MDIOCLIST ioctl.

It is unused, the ABI was broken in r322969, and it is broken by design
(more than MDNPAD md devices can exist and there is no way to retreive
them with this interface).

mdconfig(8) was converted to use libgeom to obtain this information
in r157160 and any other consumers of MDIOCLIST should likewise be
converted.

Reviewed by: emaste
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18936