[flang][openacc][NFC] Cleanup enter data lowering

Remove copyinOperands, createOperands, createZeroOperands, attachOperands
SmallVectors as they are not used anymore.
The op itself cannot be cleanup yet because
`mlir/lib/Target/LLVMIR/Dialect/OpenACC/OpenACCToLLVMIRTranslation.cpp`
still depends on it. The final clean up on the op will be down once
the translation uses the new data operand operations.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D149467