[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:

-
https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Dialect/Polynomial
-
https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Transforms/PolynomialApproximation
-
https://github.com/google/heir/tree/a422f130fa42ed2acf319a1bd6e420b651863c03/lib/Transforms/LowerPolynomialEval

Author: j2kun

mlir/include/mlir/Dialect/CMakeLists.txt

@@ -28,7 +28,6 @@ add_subdirectory(OpenACCMPCommon)
 add_subdirectory(OpenMP)
 add_subdirectory(PDL)
 add_subdirectory(PDLInterp)
-add_subdirectory(Polynomial)
 add_subdirectory(Ptr)
 add_subdirectory(Quant)
 add_subdirectory(SCF)