It's vanilla, and I think so.
Then that's why I believe.
It's vanilla, which means it's present in Skyrim.ESM. Then you delete a master record with an esp, it isn't gone. The ESP just marks it as deleted. That's why. So it's deleted and wont show up or affect anything, but it doesn't go away because it's present in the Skyrim.esm.
Plus, what are you deleting from vanilla Skyrim? That's usually bad idea.