Use different colors for Org-Mode levels (#81)

It now uses the whole "Frost" palette.
It used `nord8` for all levels before to match the style of Markdown
syntax in Emacs as well as other projects, but it makes sense for this
package to use different colors since users might expect it.
This also matches the default style of the Org-Mode package that makes
use of different colors.

Additionaly this comit removed a duplicate line.

nord-theme.el

@@ -627,14 +627,14 @@
     `(neo-vc-user-face ((,class (:foreground ,nord4))))
 
     ;; > Org
-    `(org-level-1 ((,class (:foreground ,nord8 :weight bold))))
-    `(org-level-2 ((,class (:inherit org-level-1))))
-    `(org-level-3 ((,class (:inherit org-level-1))))
-    `(org-level-4 ((,class (:inherit org-level-1))))
-    `(org-level-5 ((,class (:inherit org-level-1))))
-    `(org-level-6 ((,class (:inherit org-level-1))))
-    `(org-level-7 ((,class (:inherit org-level-1))))
-    `(org-level-8 ((,class (:inherit org-level-1))))
+    `(org-level-1 ((,class (:foreground ,nord7 :weight extra-bold))))
+    `(org-level-2 ((,class (:foreground ,nord8 :weight bold))))
+    `(org-level-3 ((,class (:foreground ,nord9 :weight semi-bold))))
+    `(org-level-4 ((,class (:foreground ,nord10 :weight normal))))
+    `(org-level-5 ((,class (:inherit org-level-4))))
+    `(org-level-6 ((,class (:inherit org-level-4))))
+    `(org-level-7 ((,class (:inherit org-level-4))))
+    `(org-level-8 ((,class (:inherit org-level-4)))))
     `(org-agenda-structure ((,class (:foreground ,nord9))))
     `(org-agenda-date ((,class (:foreground ,nord8 :underline nil))))
     `(org-agenda-done ((,class (:foreground ,nord14))))