deprecate and remove X_sparse_iter when tiledbsoma blockwise iterators are available

[bkmartinjr]

When SOMA support for blockwise iterators are released:

• [x] deprecate cellxgene_census.experimental.util.X_sparse_iter
• [x] port all uses to blockwise iterators:
  • [x] geneformer/mul
  • [x] find_dups notebook
  • [x] unit tests
  • [x] pca notebook
• [x] remove from the cellxgene_census package

Blockwise iterators will ship in TileDB-SOMA 1.5, so this needs to be deprecated in cellxgene-census version 1.8 and removed in 1.9

[atarashansky]

@atarashansky Pinging myself to stay abreast of this issue.

Downstream dependency: https://github.com/chanzuckerberg/single-cell-data-portal/issues/5973

[mlin]

I have to update the geneformer tokenizer.