remove manifests.output field from schema

- manifests.output field was introduced for users to specify kpt hydration output dir, but this the logic behind of it is never implemented.
- Specifying this filed in multiple modules project can lead to render output in different places, which are hard to manage, also there is chance that this filed in different sub-modules may override each other.
- Also, having this field for a dedicated render ``kpt`` creates more confusion.
- As the reasons above, let's remove the field from schema.