storage_proxy: remove_response_handler: assert id was found
storage_proxy: remove_response_handler: assert id was found

Help identify cases like seen in #5032 where the handler id
wasn't found from the on_down -> timeout_cb path.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla