Remove mdn_url which does not yet exist

Co-authored-by: Claas Augner <495429+caugner@users.noreply.github.com>

Author: jdatapple

css/types/random.json

@@ -4,7 +4,6 @@
       "random": {
         "__compat": {
           "description": "`random()`",
-          "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/Reference/Values/random",
           "spec_url": "https://www.w3.org/TR/css-values-5/#random",
           "support": {
             "chrome": {