Fix cider evaluation result in terminal (#71)

When running the "Cider" package [1] in Emacs GUI mode through iTerm2 on
macOS, the background and font color in the evaluation result overlay
was the same (or very similar) color so it became unreadable.
Probably that's because of defined `cider-result-overlay-face` face [2]
and a difference in the rendering engine of iTerm2.

To prevent this problem, the background color has been removed.

[1]: https://github.com/clojure-emacs/cider
[2]: https://github.com/clojure-emacs/cider/blob/f6ac1594eb20b0d3b9f9b7601e3d2708346dc0bb/cider-overlays.el#L35

Closes GH-70

nord-theme.el

@@ -639,6 +639,9 @@
     `(neo-vc-up-to-date-face ((,class (:foreground ,nord4))))
     `(neo-vc-user-face ((,class (:foreground ,nord4))))
 
+    ;; > Cider
+    `(cider-result-overlay-face ((t (:background unspecified))))
+
     ;; > Org
     `(org-level-1 ((,class (:foreground ,nord7 :weight extra-bold))))
     `(org-level-2 ((,class (:foreground ,nord8 :weight bold))))