Do not send "registration success email" for external auth sources (#24632)

Co-author: @pboguslawski 


"registration success email" is only used for notifying a user that "you
have a new account now" when the account is created by admin manually.

When a user uses external auth source, they already knows that they has
the account, so do not send such email.

Co-authored-by: Giteabot <teabot@gitea.io>

services/auth/reverseproxy.go

@@ -13,7 +13,6 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web/middleware"
-	"code.gitea.io/gitea/services/mailer"
 
 	gouuid "github.com/google/uuid"
 )
@@ -172,7 +171,5 @@ func (r *ReverseProxy) newUser(req *http.Request) *user_model.User {
 		return nil
 	}
 
-	mailer.SendRegisterNotifyMail(user)
-
 	return user
 }