refactor: remove unused .add-page-button styles for cleaner code

apps/app/src/features/openai/client/components/AiAssistant/AiAssistantManagementModal/AiAssistantManagementPageTreeSelection.module.scss

@@ -8,8 +8,4 @@
       padding: 0.4rem 0 !important;
     }
   }
-
-  .add-page-button {
-    background-color: yellow;
-  }
 }