Fix MAGN-5050 Import instance is updated in Dynamo, creates multiple instances in Revit

<h6>Summary</h6>

For ImportInstance, when the input is updated, the old instance is obtained from trace. Ideally, the old instance need to be updated according to the changed input. But there are no such functions for ImportInstance to update its geometry.

Instead the old instance can be deleted and a new instance can be created. This is safe because there are no other elements which are taking imported instances as input.

@pboyer 
PTAL
This line looks really weird to me.  Why not do the cast in `GetAllImportInstances`?