Only use supported sort order for "explore/users" page (#29430)

Thanks to inferenceus : some sort orders on the "explore/users" page
could list users by their lastlogintime/updatetime.

It leaks user's activity unintentionally. This PR makes that page only
use "supported" sort orders.

Removing the "sort orders" could also be a good solution, while IMO at
the moment keeping the "create time" and "name" orders is also fine, in
case some users would like to find a target user in the search result,
the "sort order" might help.

![image](https://github.com/go-gitea/gitea/assets/2114189/ce5c39c1-1e86-484a-80c3-33cac6419af8)

templates/explore/search.tmpl

@@ -16,8 +16,6 @@
 			<a class="{{if eq .SortType "oldest"}}active {{end}}item" href="{{$.Link}}?sort=oldest&q={{$.Keyword}}">{{ctx.Locale.Tr "repo.issues.filter_sort.oldest"}}</a>
 			<a class="{{if eq .SortType "alphabetically"}}active {{end}}item" href="{{$.Link}}?sort=alphabetically&q={{$.Keyword}}">{{ctx.Locale.Tr "repo.issues.label.filter_sort.alphabetically"}}</a>
 			<a class="{{if eq .SortType "reversealphabetically"}}active {{end}}item" href="{{$.Link}}?sort=reversealphabetically&q={{$.Keyword}}">{{ctx.Locale.Tr "repo.issues.label.filter_sort.reverse_alphabetically"}}</a>
-			<a class="{{if eq .SortType "recentupdate"}}active {{end}}item" href="{{$.Link}}?sort=recentupdate&q={{$.Keyword}}">{{ctx.Locale.Tr "repo.issues.filter_sort.recentupdate"}}</a>
-			<a class="{{if eq .SortType "leastupdate"}}active {{end}}item" href="{{$.Link}}?sort=leastupdate&q={{$.Keyword}}">{{ctx.Locale.Tr "repo.issues.filter_sort.leastupdate"}}</a>
 		</div>
 	</div>
 </div>