rpc: drop unneeded code
rpc: drop unneeded code

futurize::apply returns all exception as futures, so there is not need
to wrap it in try/catch.

Message-Id: <b289fb7ab2b69914d62c34d5688c223c78e75087@scylladb.com>

scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar