docs: fix stale references to development mode and retry target type

- README.md: remove 'in development mode' from admin user creation
  description (admin user creation is unconditional)
- internal/delivery/engine.go: remove 'and retry' from HTTPTargetConfig
  comment (retry was merged into http target type)
- internal/delivery/engine_test.go: remove '/retry' from
  newHTTPTargetConfig comment for consistency

internal/delivery/engine.go

@@ -90,7 +90,7 @@ type Notifier interface {
 	Notify(tasks []DeliveryTask)
 }
 
-// HTTPTargetConfig holds configuration for http and retry target types.
+// HTTPTargetConfig holds configuration for http target types.
 type HTTPTargetConfig struct {
 	URL     string            `json:"url"`
 	Headers map[string]string `json:"headers,omitempty"`