Remove Fomantic-UI's .hidden CSS class for menu elements (#22895)

* Like #22851
* All other dropdown menu elements do not have such `hidden` class.
* Actually the dropdown menu elements do not need it in HTML, so this PR
removes it.
* There is already `.ui.dropdown .menu { display: none; }`, so when
loading the page, the menu is correctly hidden initially, no need to add
any more CSS classes.
* The Fomantic UI's `.hidden` class should still exist until there is no
its checkbox/dropdown module anymore. The Fomantic UI JS code still
addes `hidden` for `ui checkbox` and addes `transition hidden` for `ui
menu` at the moment.
* This PR also cleans the legacy inline `style`, which is quite hacky
and no need anymore.

All these dropdown menus work well.

I have tested these 5 places:

* The code search from User Profile
* The issue sidebar to lock issue
* The repo search form
* The repo setting page: branch list
* The repo setting page: merge option list

Screenshot:

![image](https://user-images.githubusercontent.com/2114189/218534515-e7dfe291-6765-4e0b-833c-b74f17e30117.png)

templates/repo/issue/view_content/sidebar.tmpl

@@ -639,7 +639,7 @@
 
 									<div class="default text"> </div>
 
-									<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+									<div class="menu">
 										{{range .LockReasons}}
 											<div class="item" data-value="{{.}}">{{.}}</div>
 										{{end}}