[mlir][polynomial] Remove polynomial dialect (#139766)
Cf. https://discourse.llvm.org/t/future-of-the-polynomial-dialect/86117
For posterity, the polynomial dialect development has moved to HEIR, and
as of this writing the different components can be found here:
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https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Dialect/Polynomial
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https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Transforms/PolynomialApproximation
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https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Transforms/LowerPolynomialEval
