Remove unnecessary wxUSE_STL test from HTML parser benchmark

The same code can be used in STL and non-STL builds.

tests/benchmarks/htmlparser/htmlpars.cpp

@@ -367,12 +367,7 @@ void wx28HtmlParser::PopTagHandler()
     wxList::compatibility_iterator first;
 
     if ( !m_HandlersStack ||
-#if wxUSE_STL
-         !(first = m_HandlersStack->GetFirst())
-#else // !wxUSE_STL
-         ((first = m_HandlersStack->GetFirst()) == nullptr)
-#endif // wxUSE_STL/!wxUSE_STL
-        )
+         !(first = m_HandlersStack->GetFirst()) )
     {
         wxLogWarning(_("Warning: attempt to remove HTML tag handler from empty stack."));
         return;