I'm looking for a mod that takes the healing effect away from food, and I mean ONLY that. I know there are some overhaul mods like PN or Arwen's that do this, but they do also too much other stuff I don't want.. Is there one? (I guess not, but asking doesn't hurt...)

You could easily do this yourself using the Kit

...how easily? What do I have to do?

Open up the GECK, look for the ingestibles in the list on the left hand side, then right click on the food item you want to change. You'll see a line that deals with health for that item, simply delete it. When you're finished, save the mod, then activate using the mod manager of your choice.