fix(repo): unify DEFAULT_SHOW_FULL_NAME output in templates and dropdown (#36597)

The design of DefaultShowFullName has some problems, which make the UI
inconsistent, see the new comment in code

This PR does a clean up for various legacy problems, and clarify some
"user name display" behaviors.

---------

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

modules/templates/helper.go

@@ -96,9 +96,6 @@ func NewFuncMap() template.FuncMap {
 		"AssetVersion": func() string {
 			return setting.AssetVersion
 		},
-		"DefaultShowFullName": func() bool {
-			return setting.UI.DefaultShowFullName
-		},
 		"ShowFooterTemplateLoadTime": func() bool {
 			return setting.Other.ShowFooterTemplateLoadTime
 		},