[ticket/8228] Fix still existing problems with code in firefox

A user of a board mentioned that there are still some problems
in firefox if php-highlighting is on. So I used a snippet which
they worked on for weeks. Link to the snippet (in german):
http://www.ongray-design.de/forum/viewtopic.php?t=541

PHPBB3-8228

phpBB/styles/prosilver/theme/print.css

@@ -136,4 +136,4 @@ div.spacer { clear: both; }
 /* Accessibility tweaks: Mozilla.org */
 .skip_link { display: none; }
 
-dl.codebox dt { display: none; }
+.codebox p { display: none; }