kern_jail.c: Remove #ifdefs for VNET_NFSD

The consensus was that VNET_NFSD was not needed.
This patch removes it from kern_jail.c.

With this patch, support for the "allow.nfsd"
jail parameter is enabled in the kernel for
kernels built with "options VIMAGE".

(cherry picked from commit cbbb22031f9b957967c03dc8e685392f7d7e7fb8)