Consider removing `schema.id`

[lilleyse]

The schema.id property was added to uniquely identify schemas, the intent being to represent a specific data model (e.g. IFC, BIS, GGDM) in 3D Tiles, but in practice (1) schemas often mix-and-match classes from different data models (2) schemas contain supporting classes like for subtree metadata that aren't part of the data model. In both cases it's difficult to assign a meaningful id.

So with this in mind, it could make sense to treat schemas more like class libraries (as brought up in #828 (comment)) and remove schema.id.