Remove unnecessary reduce in Duration#inspect
Remove unnecessary reduce in Duration#inspect

When the `Duration` class was introduced in 276c9f29, the `parts` were
represented as an array of arrays
(for example `[[:seconds, 5], [:days, 3], [:seconds, 7]]`).
At that time the `reduce` in `#inspect` made sense,
since we would need to get the totals for each part
(the example would become `{ seconds: 12, days: 3 }`).

With the current version of `Duration` we call `to_h` on the `parts`
immediately on initialize, so now the `reduce` doesn't seem to be doing
anything meaningful.

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