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>

routers/web/repo/setting/setting.go

@@ -418,7 +418,7 @@ func SettingsPost(ctx *context.Context) {
 			Interval:      interval,
 			RemoteAddress: remoteAddress,
 		}
-		if err := repo_model.InsertPushMirror(ctx, m); err != nil {
+		if err := db.Insert(ctx, m); err != nil {
 			ctx.ServerError("InsertPushMirror", err)
 			return
 		}