I wanted to set modifier to nothing. But I end up removing the criteria.
This is my code. It removes the criteria. I wanted to make it back to nothing (not 0% but just no number)
$biddableAdGroupCriterion = new BiddableAdGroupCriterion();
$biddableAdGroupCriterion->setAdGroupId($adGroupId);
$biddableAdGroupCriterion->setCriterion($target);
$biddableAdGroupCriterionOperation = new AdGroupCriterionOperation();
$biddableAdGroupCriterionOperation->setOperand($biddableAdGroupCriterion);
$biddableAdGroupCriterionOperation->setOperator(Operator::REMOVE);
$operations[] = $biddableAdGroupCriterionOperation;
$result = $adGroupCriterionService->mutate($operations);