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".
Reviewed by: markj
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D38808