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/modules/button.css

@@ -29,12 +29,6 @@
   color: var(--color-red);
 }
 
-/* try to match button with no icons in height */
-.icon-button {
-  padding-top: 7.42px !important;
-  padding-bottom: 7.42px !important;
-}
-
 /* btn is a plain button without any opinionated styling */
 
 .btn {