Unfortunately those new features that Apple added as a dead giveaway does annoy a lot of users, like me too.
There is no way to delete pictures in Selfie or Screenshot folder, neither single pictures, nor the whole folder.
Also the proposed solution to create a new Album/folder and add Selfie pics into it, then delete all selfies in the Selfie folder,
apparently DOES NOT work. Delete icon stays greyed out and cannot be used.
So the only solution to get rid of Selfie folder and Screenshot folder is to synchronize Photos in iTunes and to delete all photos on iPhone through this action.
If you want to reintegrate the photos you have deleted before again into your iPhone, it will most likely retain the same structure as before, i.e. same Selfie folder and content, same Screenshot folder and content. So you MUST get rid of ex- or imported Photos, that are classified as Selfies and/or Screenshots, to avoid that these folders are recreated.
Anyhow, the declaration of Selfie as an Selfie by Apple seems to be sold to users as a feature, I find it utterly annoying. I myself would like to decide which folders are being created and deleted, a preconfigurated unchangeable solution by apple is not an added feature. Especially not, when it introduces more steps to manage Photos and folders and thus costs the user more valuable time. It seems that Apple has lost the view, that Steve Jobs used to have!