css: Remove `.code()` mixin

Undefined mixins don't cause the CSS to not load anymore.

public/css/icinga/compat.less

@@ -36,9 +36,3 @@ table.avp {
     background-color: #f1f1f1;
   }
 }
-
-.code() {
-  // Some 3rd party modules (e.g. vsphere) use this mixin (though we never had it)
-  // With lessphp version 0.5.0 using undefined mixins now throws exceptions
-  // Since we can't rely that users upgrade those modules first, we provide the mixin for compatibility here
-}