First, download the program, MdlVis. Once you have it all set up, open it.
After that, go to Files, and open the Warhammer Elector Count model
You'll be greeted with this:
That is the program with the opened model. To the right you have all the geosets and other stuff, but right now, you'll want to go to "Perspective" on the upper left corner. Open it, and select "Right" or "Front".
After that, you'll be facing the model directly.
Lucky for us, the first selected geoset by default includes the feathers, as well as other parts, but we only care about the feathers for now. They're also a different geoset than the Helmet itself, which makes removing the feathers much more easy. With other models, even slight edits can be complicated.
Note to the right that the default tool selected is the one that allows you to select the... vertices, I think they're called?
All you have to do is select them, just click and drag.
Once that's done, the selected vertices will be in red.
Now just press Delete on your keyboard and the feathers will be gone!
But, with the model looking like that, you can't really tell if what you did really worked, so, let's see it in game!
First of all, of course, save the model, I just went with "ElectorCountFeatherless"
Now, just import them in the editor like, I suppose, you alraedy know, and...
Voila! He does look really cool without the feathers on the armet.