NFS: Remove unused callback void decoder

[ Upstream commit c35a810c ]

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>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>