Remove incorrect null_checkexp() routine.  This will allow the NFS server to call vfs_stdcheckexp() on the exported nullfs filesystem, not the underlying filesystem being nullfs mounted. If the lower filesystem was not NFS exported, then the NFS exported null filesystem would not work.
Remove incorrect null_checkexp() routine.  This
will allow the NFS server to call vfs_stdcheckexp() on the exported nullfs
filesystem, not the underlying filesystem being nullfs mounted.
If the lower filesystem was not NFS exported, then the NFS exported
null filesystem would not work.

Pointed out by:	scottl
PR:		kern/87906
MFC after:	1 week

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