First of all, "Enchanted creature has protection from white" is an ability, not an effect. This ability creates a continuous effect that grants the enchanted creature protection from white. This is the effect to which the card refers.
...Except it's not. That would be the case is White Ward was written correctly. The problem is that this effect isn't the one that would cause the White Ward to be removed. It's actually the effect created by the granted "Protection from White" ability that would cause the White Ward to be removed. So obviously, that's the effect to which the card actually refers.
Without neutering the Protection from White it grants, White Ward would protect the creature from being enchanted by White Ward, sending White Ward to the graveyard.
Of course, it only neuters the Protection from White it grants. If the creature has any other instance of Protection from White, those will still apply.