messaging: remove default when computing rpc client index
messaging: remove default when computing rpc client index

A default means that when adding new verbs, we may forget to
categorize a verb correctly.  Without the default, the compiler
will complain due to -Wswitch.

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