RNS::ExtractSliceOp is unused

[crockeea]

I could not find any uses of RNS::ExtractSliceOp. Consider removing it.

[j2kun]

In theory, rns.extract_slice combined with polynomial.apply_coefficientwise could replace polynomial.extract_slice. Do you think that's worthwhile?

I noticed that polynomial.extract_slice has no current lowerings, so at least, going through apply_coefficientwise + rns.extract_slice may provide a slightly easier path to LLVM, even if we want to keep polynomial.extract_slice to target a primitive on an accelerator.