Clean template/helper.go (#23922)

The first step of #23328

This PR cleans:

1. Dead function like `SubStr`
2. Unnecessary function like `UseHTTPS`, it should be provided by
`window.origin`
3. Duplicate function like `IsShowFullName`, there was already a
`DeafultShowFullName`

I have searched these removed functions globally, no use in code.

templates/admin/config.tmpl

@@ -11,7 +11,7 @@
 				<dt>{{.locale.Tr "admin.config.app_name"}}</dt>
 				<dd>{{AppName}}</dd>
 				<dt>{{.locale.Tr "admin.config.app_ver"}}</dt>
-				<dd>{{AppVer}}{{AppBuiltWith}}</dd>
+				<dd>{{AppVer}}{{.AppBuiltWith}}</dd>
 				<dt>{{.locale.Tr "admin.config.custom_conf"}}</dt>
 				<dd>{{.CustomConf}}</dd>
 				<dt>{{.locale.Tr "admin.config.app_url"}}</dt>