rpc: remove no longer used template argument.
rpc: remove no longer used template argument.

syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar