Don't explicitly set MLIR_PDLL_TABLEGEN_EXE

[marbre]

With llvm/llvm-project@91b6f76, the variable MLIR_PDLL_TABLEGEN_EXE is
set as a cache variable in MLIR upstream.

Likely requires an update of externals/mlir-hlo to
tensorflow/mlir-hlo@4f2a00b or later.

[jpienaar]

Change looks good, are you going to wait until the bump happens? (I'm assuming no presubmits fail here for related reason as this is only enabled for the MHLO and that's not yet part of presubmit).

[powderluv]

Thank you

[marbre]

Change looks good, are you going to wait until the bump happens? (I'm assuming no presubmits fail here for related reason as this is only enabled for the MHLO and that's not yet part of presubmit).

I know that a similar fix "broke" IREE but I think in that case the underlying LLVM-integrate was missing. That is not the case here (the LLVM version used includes llvm/llvm-project@91b6f76) though we could probably merge without breaking anything. However, the change might only fully functional after MLIR-HLO is up-to-date enough as well.