Don't set markdown-code-face background

Setting the background based on the theme at load time (which is a
light backround if a theme other than `default` has not been loaded
yet) produces undesirable results if the user later switches to a
theme with a dark backround that has not defined `markdown-code-face`.

Closes GH-273.

markdown-mode.el

@@ -2621,16 +2621,10 @@ See `markdown-hide-markup' for additional details."
   :group 'markdown-faces)
 
 (defface markdown-code-face
-  (let ((default-bg (face-background 'default))
-        light-bg dark-bg)
-    (if (member default-bg '(nil unspecified "unspecified-bg" "SystemWindow"))
-        (setq light-bg "unspecified-bg" dark-bg "unspecified-bg")
-      (setq light-bg (color-darken-name default-bg 3)
-            dark-bg (color-lighten-name default-bg 3)))
-    `((default :inherit fixed-pitch)
-      (((type graphic) (class color) (background dark)) (:background ,dark-bg))
-      (((type graphic) (class color) (background light)) (:background ,light-bg))))
-  "Face for inline code, pre blocks, and fenced code blocks."
+  '((t (:inherit fixed-pitch)))
+  "Face for inline code, pre blocks, and fenced code blocks.
+This may be used, for example, to add a contrasting background to
+inline code fragments and code blocks."
   :group 'markdown-faces)
 
 (defface markdown-inline-code-face