NFS: Remove unused callback void decoder

Clean up: The callback RPC dispatcher no longer invokes these call
outs, although svc_process_common() relies on seeing a .pc_encode
function.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/callback_xdr.c

@@ -63,11 +63,10 @@ static __be32 nfs4_callback_null(struct svc_rqst *rqstp)
 	return htonl(NFS4_OK);
 }
 
-static int nfs4_decode_void(struct svc_rqst *rqstp, __be32 *p)
-{
-	return xdr_argsize_check(rqstp, p);
-}
-
+/*
+ * svc_process_common() looks for an XDR encoder to know when
+ * not to drop a Reply.
+ */
 static int nfs4_encode_void(struct svc_rqst *rqstp, __be32 *p)
 {
 	return xdr_ressize_check(rqstp, p);
@@ -1067,7 +1066,6 @@ static struct callback_op callback_ops[] = {
 static const struct svc_procedure nfs4_callback_procedures1[] = {
 	[CB_NULL] = {
 		.pc_func = nfs4_callback_null,
-		.pc_decode = nfs4_decode_void,
 		.pc_encode = nfs4_encode_void,
 		.pc_xdrressize = 1,
 		.pc_name = "NULL",