I got this idea from KNGR's Deserts of Novigrad mod. To remove the foliage you need to remove a certain folder/files in The Witcher 3 directory, which I will specify in the install directions below. What my actual mod does is disable the collisions with trees. Deleting the shader folder/files will simply make trees invisible, subjecting you to random and annoying collisions whenever you go off the beaten path. Not gonna be a problem anymore. Mostly. This will obviously give you a big fps boost and that may be the main reason you're installing this mod.

Known Issues:
Apparently this doesn't affect Roach for the most part. I tried to have Roach not collide into the invisible trees, but I guess she's built different. And not in a good way.
Unknown Issues:
There are probably a lot but the one above is the only one I found when testing it. I'm not actually going to be using this mod so yeah.

Installation:
1. I'd recommend checking out Zinn's mod No trees and (or) grass if you want more details on which specific files to remove but I'll just put the TLDR here
2. Go to your game directory: The Witcher 3 Wild Hunt GOTY\bin\shaders\speedtree\shaders_directx11
3. You will now be deleting files from the folder shaders_directx11.
   a. If you want to remove all foliage, delete all files except the ones starting with billboard.
   b. If you want to only remove grass, delete all files that start with grass.
   c. If you want to remove all trees, delete all files that have the words "leaves" or "fronds and caps"
4. Install my mod just like any other mod, which will remove all foliage collisions except for Roach.


You can find my other mods for The Witcher 3 here:
Customizable Colors for Aerondight Glow Effect
Customizable Colors for Sword Trails
Master Sword from the Legend of Zelda
Lore-Friendly Dilated Pupils for Modular Eyes
TW2 Triss Blue Retexture
Daedric Dagger Knife Replacement