[mlir] Remove unused variable 'kCopyFlag' in OpenACCToLLVMIRTranslation.cpp (NFC)

/data/llvm-project/mlir/lib/Target/LLVMIR/Dialect/OpenACC/OpenACCToLLVMIRTranslation.cpp:41:27: error: unused variable 'kCopyFlag' [-Werror,-Wunused-const-variable]
static constexpr uint64_t kCopyFlag = kDeviceCopyinFlag | kHostCopyoutFlag;
                          ^
1 error generated.