Assistant: Use static_cast instead of dynamic_cast

When the type is guaranteed to be known, there's no need to use RTTI.

Userland/Applications/Assistant/main.cpp

@@ -206,7 +206,7 @@ int main(int argc, char** argv)
 
  auto mark_selected_item = [&]() {
  for (size_t i = 0; i < app_state.visible_result_count; ++i) {
- auto& row = dynamic_cast<Assistant::ResultRow&>(results_container.child_widgets()[i]);
+ auto& row = static_cast<Assistant::ResultRow&>(results_container.child_widgets()[i]);
  row.set_is_highlighted(i == app_state.selected_index);
  }
  };