Merge db.Iterate and IterateObjects (#21641)

These two functions are similiar, merge them.

modules/doctor/misc.go

@@ -30,10 +30,9 @@ import (
 func iterateRepositories(ctx context.Context, each func(*repo_model.Repository) error) error {
 	err := db.Iterate(
 		ctx,
-		new(repo_model.Repository),
 		builder.Gt{"id": 0},
-		func(idx int, bean interface{}) error {
-			return each(bean.(*repo_model.Repository))
+		func(ctx context.Context, bean *repo_model.Repository) error {
+			return each(bean)
 		},
 	)
 	return err