Remove support for sideloading

[bbatsov]

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?

[bbatsov]

The code in question is mostly here https://github.com/clojure-emacs/cider/blob/master/cider-eval.el#L208

[alexander-yakushev]

add-middleware still exists in nREPL, but I guess it's of no use here in CIDER without the sideloading. I'd say nixing it and restoring in the future is a reasonable way to go.

[bbatsov]

Agreed.