nfsd: Delete nfsrv_prison_cleanup() until vneting enabled

Oops, although the vneting macros do not do anything yet,
commit 7344856e3a6d enabled the prison cleanup function, that
would get called and crash the system when a jail was terminated.

This patch gets rid of nfsrv_prison_cleanup() for now.
It can go in when the vnet macros are enabled as
front ends to the vnet macros.

MFC after: 3 months