Remove eval'd clojure code in favor of cider's cider-dispatch-complete-symbol

[gtrak]

https://github.com/clojure-emacs/cider/blob/master/cider-interaction.el#L582

This falls back to eval if not present, but will now rely on the presence of the 'complete' nrepl op, which also works with clojurescript repls.

Backend impl is here: https://github.com/gtrak/cider-nrepl/blob/master/src/cider/nrepl/middleware/complete.clj

Company mode is doing something similar: https://github.com/clojure-emacs/company-cider/blob/master/company-cider.el#

Seems sensible to me since ac-nrepl's already relying on cider.