Unify and simplify TrN for i18n (#18141)

Refer: https://github.com/go-gitea/gitea/pull/18135#issuecomment-1003246099

Now we have a unique and simple `TrN`, and make the fix of PR #18135 also use the better `TrN` logic.

services/mailer/mail.go

@@ -78,7 +78,6 @@ func sendUserMail(language string, u *user_model.User, tpl base.TplName, code, s
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var content bytes.Buffer
@@ -129,7 +128,6 @@ func SendActivateEmailMail(u *user_model.User, email *user_model.EmailAddress) {
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var content bytes.Buffer
@@ -160,7 +158,6 @@ func SendRegisterNotifyMail(u *user_model.User) {
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var content bytes.Buffer
@@ -194,7 +191,6 @@ func SendCollaboratorMail(u, doer *user_model.User, repo *repo_model.Repository)
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var content bytes.Buffer
@@ -278,7 +274,6 @@ func composeIssueCommentMessages(ctx *mailCommentContext, lang string, recipient
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var mailSubject bytes.Buffer

services/mailer/mail_release.go

@@ -76,7 +76,6 @@ func mailNewRelease(lang string, tos []string, rel *models.Release) {
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	var mailBody bytes.Buffer

services/mailer/mail_repo.go

@@ -71,7 +71,6 @@ func sendRepoTransferNotifyMailPerLang(lang string, newOwner, doer *user_model.U
 		// helper
 		"i18n":     locale,
 		"Str2html": templates.Str2html,
-		"TrN":      templates.TrN,
 	}
 
 	if err := bodyTemplates.ExecuteTemplate(&content, string(mailRepoTransferNotify), data); err != nil {