just remove or erase the file image (not delete) is all you need to do.
page 2 has a bit more info about removing film grain...
Common Modified files
...or you can just download this mod for it. which is the same thing as above, just more user friendly since the work has been done and all you have to do is replace the file.
Rock3t's remove film grain
@lombra well there is one other part that you might want to remove as well. the lens flare or specks.
i made a quick zip file of the removed lens flare/specks incase you do not want to remove it yourself. i uploaded it to mediafire. which if you have an account there you will not get ads or anything. however you will get ads if you are not signed in or do not have an account. this is just the one file and copy/replace the original texture.zip image with this one, the same way as the film grain removal is replaced.
remove lens flare/specks