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/source/ldap/source_authenticate.go

@@ -14,7 +14,6 @@ import (
 	auth_module "code.gitea.io/gitea/modules/auth"
 	"code.gitea.io/gitea/modules/util"
 	source_service "code.gitea.io/gitea/services/auth/source"
-	"code.gitea.io/gitea/services/mailer"
 	user_service "code.gitea.io/gitea/services/user"
 )
 
@@ -100,8 +99,6 @@ func (source *Source) Authenticate(user *user_model.User, userName, password str
 			return user, err
 		}
 
-		mailer.SendRegisterNotifyMail(user)
-
 		if isAttributeSSHPublicKeySet && asymkey_model.AddPublicKeysBySource(user, source.authSource, sr.SSHPublicKey) {
 			if err := asymkey_model.RewriteAllPublicKeys(); err != nil {
 				return user, err