remove procfs ctl interface

This interface has no in-tree consumers and has been more or less
non-functional for several releases.

Remove manpage note that the procfs special file 'mem' is grouped to
kmem. This hasn't been true since r81107.

Remove procfs' README file. It is an out of date duplication of the manpage
(quoth the README: "since the bsd kernel is single-processor...").

Reviewed by: vangyzen, bcr (manpage)
Approved by: des (procfs maintainer), vangyzen (mentor)
Differential Revision: https://reviews.freebsd.org/D9802