fix(timeutil): remove unreachable panic() call

[thp-canonical]

Stumbled upon this while looking for

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

• https://github.com/canonical/snapd/pull/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

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Related proposed downstream Pebble patch: https://github.com/canonical/pebble/pull/455