Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

modules/web/middleware/locale.go

@@ -51,9 +51,3 @@ func Locale(resp http.ResponseWriter, req *http.Request) translation.Locale {
 func SetLocaleCookie(resp http.ResponseWriter, lang string, maxAge int) {
 	SetSiteCookie(resp, "lang", lang, maxAge)
 }
-
-// DeleteLocaleCookie convenience function to delete the locale cookie consistently
-// Setting the lang cookie will trigger the middleware to reset the language to previous state.
-func DeleteLocaleCookie(resp http.ResponseWriter) {
-	SetSiteCookie(resp, "lang", "", -1)
-}