Fix TODO for "deleted" support in LTIX

[csev]

Per Stack Overflow

https://stackoverflow.com/questions/44474250/which-is-better-in-mysql-an-ifnull-or-or-logic/44474286

I should not use NULL as equivalent of zero in a pseudo-boolean field.

https://github.com/tsugiproject/tsugi-php/blob/master/src/Core/LTIX.php#L682

But I want to delay breaking all of Tsugi for a while - so come back to this in a few months.