NFSD: remove unused structs 'nfsd3_voidargs'

'nfsd3_voidargs' in nfs[23]acl.c is unused since
commit 788f7183

 ("NFSD: Add common helpers to decode void args and
encode void results").

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>