storage_proxy: remove unneeded continuation
storage_proxy: remove unneeded continuation

make_ready_future() around when_all() is not longer needed. It was
added to catch mutate_locally() exceptions, but now it is handled in
lower level.

duarten/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla