PR #57157: Don't set MLIR_PDLL_TABLEGEN_EXE

Imported from GitHub PR tensorflow/tensorflow#57157

With llvm/llvm-project@91b6f76 landed, external projects no longer need
to set `MLIR_PDLL_TABLEGEN_EXE`. The variable is now set as a cache
variable in MLIR upstream. Setting it in an external project breaks
cross-compilation.

This patch is similar to 77bd8ec.
Copybara import of the project:

--
54ae3dfd45fe47751e1400385377074d0ce7c0fe by Marius Brehler <marius.brehler@iml.fraunhofer.de>:

Don't set MLIR_PDLL_TABLEGEN_EXE

With llvm/llvm-project@91b6f76 landed, external projects no longer need
to set `MLIR_PDLL_TABLEGEN_EXE`. The variable is now set as a cache
variable in MLIR upstream. Setting it in an external project breaks
cross-compilation.

This patch is similar to 77bd8ec.

Merging this change closes #57157

PiperOrigin-RevId: 467945431

CMakeLists.txt

@@ -95,7 +95,6 @@ if(MHLO_EXTERNAL_PROJECT_BUILD)
   set(MLIR_MAIN_SRC_DIR ${LLVM_MAIN_SRC_DIR}/../mlir ) # --src-root
   set(MLIR_INCLUDE_DIR ${MLIR_MAIN_SRC_DIR}/include ) # --includedir
   set(MLIR_GENERATED_INCLUDE_DIR ${LLVM_BINARY_DIR}/tools/mlir/include)
-  set(MLIR_PDLL_TABLEGEN_EXE $<TARGET_FILE:mlir-pdll>)
   include_directories(SYSTEM ${MLIR_INCLUDE_DIR})
   include_directories(SYSTEM ${MLIR_GENERATED_INCLUDE_DIR})
   include_directories(SYSTEM ${MLIR_TABLEGEN_OUTPUT_DIR})