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/pam/source_authenticate.go

@@ -12,7 +12,6 @@ import (
 	"code.gitea.io/gitea/modules/auth/pam"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
-	"code.gitea.io/gitea/services/mailer"
 
 	"github.com/google/uuid"
 )
@@ -67,8 +66,6 @@ func (source *Source) Authenticate(user *user_model.User, userName, password str
 		return user, err
 	}
 
-	mailer.SendRegisterNotifyMail(user)
-
 	return user, nil
 }