Bug 23463: (follow-up) Fix timestamp default value

Bug 23463: (follow-up) Fix timestamp default value

For discussion, this patch revert the changes made previously.

This line exists in Koha::Item->store as it is the translation of:
    if (exists $item->{'timestamp'}) {
        delete $item->{'timestamp'};
    }
that was coming from _do_column_fixes_for_mod (called from ModItem)

To preserve existing behavior I would be in favor of keeping it like that to
avoid regression, and deal with it separately if we want to improve/remove it.

So basically here we are setting it to undef in Koha::Item->store to make it
handle correctly by the parent Koha::Object->store. I agree that's kind of weird
and must be improved.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>