feat(web): drop redundant preview pill from topbar

reaction_board/web/static/app.js

@@ -158,9 +158,6 @@ async function boot() {
     };
     if (data?.preview) {
       banner.hidden = false;
-      document.querySelectorAll(".topbar-preview").forEach((element) => {
-        element.hidden = false;
-      });
     }
   } catch {
     toast("无法连接看板服务", "error");