fix(timeutil): remove unreachable panic() call

[thp-canonical]

This has been added in upstream snapd back in the day here:

• timeutil: timeutil.Human(t) gives a human-friendly string for t snapd#4775

Move conditionals to the correct case where they belong (e.g. >= -cutoffDays belongs to the "%d days ago", ...), and then just naturally move the non-special case of then.Format("2006-01-02") to the default case.

Looking at compiler explorer, it seems like Go (gc 1.21 on x86_64) can optimize out the old version's panic(), so it's just dead code in the source (as it should be): https://godbolt.org/z/fxqTzYfPv

[benhoyt]

Does this actually cause any problems? I'm not opposed to this, though might be better to get it into snapd first? Otherwise it seems to me "if it ain't broke, don't fix it".

[thp-canonical]

Proposed fix in snapd: canonical/snapd#14427