Fix dropdown icon layout on diff page (#25397) (#25403)

Backport #25397 by @wxiaoguang

Address
https://github.com/go-gitea/gitea/pull/25163#issuecomment-1599207916

Remove the unused  "icon-button".

And fix the layout:

Without the dropdown icon:

```
	{{svg "gitea-whitespace"}}
```


![image](https://github.com/go-gitea/gitea/assets/2114189/58a524ba-f289-4982-aea2-6f9f9f9cbdcf)


With the dropdown icon:

```
	{{svg "gitea-whitespace" 16 "gt-mr-3"}}
	{{svg "octicon-triangle-down" 14 "dropdown icon"}}
```


![image](https://github.com/go-gitea/gitea/assets/2114189/eb99168b-5d49-40a7-8665-5296cbb4e486)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/repo.css

@@ -1666,8 +1666,9 @@
   background-color: var(--color-teal);
 }
 
-.repository .diff-detail-box .button {
-  padding: 8px 12px;
+.repository .diff-detail-box .ui.button {
+  padding: 0 12px;
+  height: 30px;
 }
 
 .repository .diff-box .header:not(.resolved-placeholder) {