Dashboard context dropdown position fix on landing page in mobile view. (#27047)

as title.

Screensots

before 

![image](https://github.com/go-gitea/gitea/assets/80308335/d72da379-1fb1-4d75-9f3e-f70e06ad4065)

after

![image](https://github.com/go-gitea/gitea/assets/80308335/110ea806-feed-4947-bf56-2985b1e1ec5f)

web_src/css/dashboard.css

@@ -89,9 +89,3 @@
 .dashboard .dashboard-navbar .ui.dropdown {
   max-width: 100%;
 }
-
-@media (max-width: 767.98px) {
-  .dashboard .dashboard-navbar .ui.dropdown > .menu {
-    position: static;
-  }
-}