`Accessor` implementation not conformant with specification about gltf

[expenses]

I just encountered this when trying to implement MSFT_lod with #348. The exact wording of the spec is:

The index of the buffer view. When undefined, the accessor MUST be initialized with zeros; sparse property or extensions MAY override zeros with actual values.

Therefore these lines should be removed:

gltf/gltf-json/src/accessor.rs

Lines 233 to 237 in 51d8f5c

	if self.sparse.is_none() && self.buffer_view.is_none() {
	// If sparse is missing, then bufferView must be present. Report that bufferView is
	// missing since it is the more common one to require.
	report(&|| path().field("bufferView"), Error::Missing);
	}