[mlir][openacc] Cleanup acc.serial from old data clause operands

Remove old clause operands from acc.serial operation since
the new dataOperands is now in place.
private and firstprivate will receive some redesign but are
not part of the new dataOperands.

Depends on D150207

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D150224