Clean up legacy top-level Configuration.Concurrency field and dead `m set` command

While reviewing #667, we noticed some vestigial code from before per-service concurrency was introduced:

• cli/commands/set.go defines a Set command (m set -c <n>) that writes to Configuration.Concurrency, but it's not registered in the dispatch table — dead code
• cli/commands/app_status.go:81 checks appConfig.HasConcurrency() on that same top-level field, which is never populated by GetConfiguration — the check always evaluates to false
• The Configuration.concurrency and auto_concurrency fields in api/app/rpc.yml (indices 1 and 2) may also be candidates for removal if nothing else references them

Low priority, just tidying up so the next person reading this code doesn't have to figure out why it's there.