Refactor deletion (#28610)

Introduce the new generic deletion methods
- `func DeleteByID[T any](ctx context.Context, id int64) (int64, error)`
- `func DeleteByIDs[T any](ctx context.Context, ids ...int64) error`
- `func Delete[T any](ctx context.Context, opts FindOptions) (int64,
error)`

So, we no longer need any specific deletion method and can just use
the generic ones instead.

Replacement of #28450

Closes #28450

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/org.go

@@ -57,7 +57,7 @@ func RemoveOrgUser(ctx context.Context, orgID, userID int64) error {
 	}
 	defer committer.Close()
 
-	if _, err := db.GetEngine(ctx).ID(ou.ID).Delete(ou); err != nil {
+	if _, err := db.DeleteByID[organization.OrgUser](ctx, ou.ID); err != nil {
 		return err
 	} else if _, err = db.Exec(ctx, "UPDATE `user` SET num_members=num_members-1 WHERE id=?", orgID); err != nil {
 		return err