feat(bot): remove hacky fix for extremely slow system deletes

they are no longer extremely slow

PluralKit.Core/Database/Repository/ModelRepository.System.cs

@@ -144,7 +144,6 @@ public partial class ModelRepository
 
     public async Task DeleteSystem(SystemId id)
     {
-        await _db.Execute(c => c.QueryAsync("update systems set is_deleting = true where id = @id", new { id = id }));
         var query = new Query("systems").AsDelete().Where("id", id);
         await _db.ExecuteQuery(query);
         _logger.Information("Deleted {SystemId}", id);