Removing scene objects can only deactivate them

[ChengshuLi]

We will still see the prim on the current stage, but it will be grayed out. This is the best we can do when removing an object that is originally in the scene json because we pre-baked all the objects into a monolithic USD before loading it.

And unfortunately we cannot add a new prim at this path afterwards, which can introduce some unexpected behavior (e.g. if we save the scene with og.sim.save, and then remove a scene object, og.sim.restore won't be able to incrementally import back the original object).

Potential TODOs:

1. print out an error if the user tries to import an object at an deactivated prim path
2. og.sim.restore() should restore from scratch if this error is detected