[gfm mode] Remove link to nonexistent stylesheet

Author: marijnh

mode/gfm/index.html

@@ -16,7 +16,6 @@
     <script src="../htmlmixed/htmlmixed.js"></script>
     <script src="../clike/clike.js"></script>
 
-    <link rel="stylesheet" href="../markdown/markdown.css">
     <style type="text/css">.CodeMirror {border-top: 1px solid black; border-bottom: 1px solid black;}</style>
     <link rel="stylesheet" href="../../doc/docs.css">
   </head>