lockd: Remove unused typedef

Clean up: Looks like the last usage of this typedef was removed by
commit 026fec7e ("sunrpc: properly type pc_decode callbacks") in
2017.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>