Remove the startup hypervisor identity backfill

The backfill existed to persist identity tokens for legacy metadata so
destructive paths could skip socket resolution. Correctness never
depended on it: a token-less live PID is confirmed through the
expected-owner fast path (one /proc/net/unix read plus one fd-dir read),
and after a reboot the empty listener scan classifies the recorded
hypervisor as provably dead. Stop, delete, and standby re-resolve
ownership on every call regardless, and instances mint tokens on
create/restore, so the startup pass and its metadata rewrites bought
nothing.

Author: yummybomb

cmd/api/main.go

@@ -296,11 +296,6 @@ func run() error {
 	}); ok {
 		reconciler.StartTAPGCReconciler(ctx)
 	}
-	if backfiller, ok := app.InstanceManager.(interface {
-		BackfillHypervisorProcessIdentities(context.Context)
-	}); ok {
-		go backfiller.BackfillHypervisorProcessIdentities(ctx)
-	}
 
 	// Log OTel status
 	if cfg.Otel.Enabled {