Remove the is_seurat_convertible return from cellxgene-schema

[joyceyan]

We stop using the return value in data portal here: #7380

In the schema 5.3 release of the validator, we'll need to remove this:
https://github.com/search?q=repo%3Achanzuckerberg%2Fsingle-cell-curation%20is_seurat_convertible&type=code

We no longer need to return this in the validator, so we can stop calculating it and also stop returning it.