Sideloading was removed in nREPL 1.3 and we should clean up CIDER's code accordingly. The only open issue is whether to delete both sideloading and add-middleware or only sideloading code, as they are kind of related and add-middleware won't work anyways if we kept it. So, probably we should just kill everything and restore add-middleware if we ever get it to work on nREPL's side. @alexander-yakushev What do you think about this?