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/sspi_windows.go

@@ -19,7 +19,6 @@ import (
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/services/auth/source/sspi"
-	"code.gitea.io/gitea/services/mailer"
 
 	gouuid "github.com/google/uuid"
 	"github.com/quasoft/websspi"
@@ -191,8 +190,6 @@ func (s *SSPI) newUser(username string, cfg *sspi.Source) (*user_model.User, err
 		return nil, err
 	}
 
-	mailer.SendRegisterNotifyMail(user)
-
 	return user, nil
 }