Fix height for sticky head on large screen on PR page (go-gitea#23111)

Right now on the PR 'File Change' Tab, the file title header sticky to
the top on large screens has wrong height, resulting in wrong ui
behavior when scrolling down. This PR is to fix this.

Before:

<img width="964" alt="截屏2023-02-24 17 12 29"
src="https://user-images.githubusercontent.com/17645053/221140409-025c4a84-6bbe-4b5b-a13f-bd2b79063522.png">

After:
<img width="1430" alt="截屏2023-02-24 21 10 12"
src="https://user-images.githubusercontent.com/17645053/221186750-0344d652-4610-4a90-a4c0-7f6269f950d6.png">

web_src/less/_repository.less

@@ -3218,17 +3218,9 @@ td.blob-excerpt {
 .ui.attached.header.diff-file-header {
   &.sticky-2nd-row {
     position: sticky;
-    top: 46px;
+    top: 77px;
     z-index: 7;
-
-    @media @mediaMd {
-      top: 77px;
-    }
-
-    @media @mediaSm {
-      top: 77px;
-    }
-
+
     @media (max-width: 480px) {
       position: static;
     }