The Dremora get their face tattoo from their head texture. Under TextureSet, look up SkinHeadMaleDremora (or FemaleDremora). Highlight the Detail Map and you'll see it.
Removing that detail map will clear the tattoo, but for every Dremora in the game. So if you only want to do it for your summoned NPC, then I would suggest you duplicate the DremoraRace, as that's where it's defined. Duplicate the TextureSet, clear the detail map (or add your own new one). In the Race record, under the Tinting tab, down in the bottom left in the Face Details Texture Set List. Point it to your new TextureSet.
I have a tutorial on http://tesalliance.org/forums/index.php?/tutorials/article/156-creating-and-adding-new-warpaints/ that might help, though that is slightly different than detail maps.