[poly]: Use a default builder to make the encoding parameter optional

[j2kun]

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Right now poly.poly has a custom builder to get the defaults working properly, but tablegen is supposed to have native support for default valued attributes. I couldn't get it to work. See PolyTypes.td

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This issue has 1 outstanding TODOs:

• lib/Dialect/Polynomial/IR/PolynomialTypes.td:30: Skip the default builder since the custom builder enables the

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[j2kun]

This has been removed in the upstream port of Polynomial, and from what I can tell it is unused in the codebase. I'm going to remove it for now so we can rethink how to track the encoding type across the IR. I think it might make more sense to use NTT/INTT for this.