fix migration 78 error mssql (#5791)

models/migrations/v78.go

 	var err error
 	if models.DbCfg.Type == core.POSTGRES || models.DbCfg.Type == core.SQLITE {
 		_, err = sess.Exec("DROP INDEX IF EXISTS IDX_repository_is_bare")
+	} else if models.DbCfg.Type == core.MSSQL {
+		_, err = sess.Exec("DROP INDEX IF EXISTS IDX_repository_is_bare ON repository")
 	} else {
 		_, err = sess.Exec("DROP INDEX IDX_repository_is_bare ON repository")
 	}
 		return err
 	}
 
-	// Then reset the values
-	sess = x.NewSession()
-	defer sess.Close()
 	if err := sess.Begin(); err != nil {
 		return err
 	}