Remove no longer existing `all_day` JSON field

[wizeman]

The all_day field no longer exists on Todoist sync API's Items.

As a consequence, due dates that were newly synced were always being shown as date+time, where time was always 0:00, even if the due date had no associated time.

Instead of relying on Todoist's API, this PR calculates the equivalent of all_day based on how the date is parsed.

[wizeman]

@sachaos I've fixed the issue you mentioned. However, I think I found a regression.

When I do todoist list -f today, some tasks that are due today do not show up in the output. Probably these are tasks that have times attached.

So I don't suggest merging this as-is.

[dwainm]

@wizeman I'll take a look at this shortly. Could you help me replicate this?