vfs: retire KERN_VNODE

It got disabled in 2003:

commit acb18acfec97aa7fe26ff48f80a5c3f89c9b542d
Author: Poul-Henning Kamp <phk@FreeBSD.org>
Date: Sun Feb 23 18:09:05 2003 +0000

Bracket the kern.vnode sysctl in #ifdef notyet because it results
in massive locking issues on diskless systems.

It is also not clear that this sysctl is non-dangerous in its
requirements for locked down memory on large RAM systems.

There does not seem to be practical use for it and the disabled routine
does not work anyway.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D39127