LibWeb: Remove outdated comment about definite calc() sizes

We already do determine whether calc() sizes are definite, so this
comment was not necessary.

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -466,7 +466,6 @@ void LayoutState::UsedValues::set_node(NodeWithStyle& node, UsedValues const* co
             }
             return false;
         }
-        // FIXME: Determine if calc() value is definite.
         return false;
     };