LibGUI: Remove Button& parameter from Button::on_click hook

There was but a single user of this parameter and it's a bit tedious
to write it out every time, so let's get rid of it.

DevTools/HackStudio/FindInFilesWidget.cpp

@@ -147,7 +147,7 @@ FindInFilesWidget::FindInFilesWidget()
         current_editor().set_focus(true);
     };
 
-    m_button->on_click = [this](auto&) {
+    m_button->on_click = [this] {
         auto results_model = find_in_files(m_textbox->text());
         m_result_view->set_model(results_model);
     };