portico: Remove obsolete content: none markdown CSS.

This made it impossible to e.g. use Font Awesome icons inside a `<ul>`
list item (they worked correctly inside `<ol>` list items).

This line was apparently added in
17ad591eb4.  The original thinking
behind this line is not clear in the original PR, but is likely a
forgotten relic from experiments with a custom unordered list bullet
styling.

static/styles/portico/markdown.css

@@ -173,10 +173,6 @@
 
         & > li {
             margin: 5px 0;
-
-            ::before {
-                content: none;
-            }
         }
     }