Grass Overhaul 5.2 includes a "remove" directory that tells you what files to remove.

Do I need to unpack the vanilla/expansion/DLC BSA's to make sure the files are removed?

Or do I just need to remove them from 3rd-party mods like Qarl's Texture Pack III?

Thanks.

remove folder contains fixes for a constant landscape blood flickering problem.

Removing
\textures\landscape\default.dds

\textures\landscape\default_n.dds

from Grass Overhault 5.2 core mod, fix this problem (that I have reported in tesnexus mod comments).

but the mod author was not clear about this problem and didn't include any explanation text about the role of this "remove" folder.
He also was not clear about Wrye Bash tag to his mod. An unofficial BOSS masterfile release in tesnexus suggest to tag the mod with

Deactivate
Graphics
NoMerge

which is odd for me since this mod can't be merged in bashed patch in anyway. Deactivating it is only optional.