Don't use LLVM_TABLEGEN_FLAGS with mlir-pdll: it's not a TableGen tool (#67486)

This can lead to build failure when a project is customizing this flag
for TableGen. This seems to have been copy/pasted from TableGen CMake
functions.

Author: joker-eph

mlir/cmake/modules/AddMLIR.cmake

@@ -108,7 +108,6 @@ function(_pdll_tablegen project ofn)
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${tablegen_exe} ${ARG_UNPARSED_ARGUMENTS} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${tblgen_includes}
-    ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     ${tblgen_change_flag}
     ${additional_cmdline}