KHR_draco_mesh_compression: Clarify required status of attributes

If I understood correctly:
If `KHR_draco_mesh_compression` is defined in the `extensions_required` field, only compressed data should be kept.

Therefore, `indices` and `attributes` of the primitive should be omitted (as well as the referenced bufferviews and buffers if they are not used elsewhere). To make this possible, `attributes` should no longer be required or should not have the requirement to consist of minimum one item, if `KHR_draco_mesh_compression` is defined in the primitive.

If this is the desired functionality, it would be a good idea to explicitly state it in the documentation of `KHR_draco_mesh_compression`

Read the KHR_draco_mesh_compression documentation and the primitive requirements for attributes and indices. Resolve whether compressed primitives may omit those fields and their unused buffer references, then update the extension documentation to state the agreed requirement explicitly.

Written by the indexing model from the issue text.