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.

activesupport/lib/active_support/duration.rb

@@ -373,7 +373,6 @@ module ActiveSupport
       return "0 seconds" if parts.empty?
 
       parts.
-        reduce(::Hash.new(0)) { |h, (l, r)| h[l] += r; h }.
         sort_by { |unit,  _ | PARTS.index(unit) }.
         map     { |unit, val| "#{val} #{val == 1 ? unit.to_s.chop : unit.to_s}" }.
         to_sentence(locale: ::I18n.default_locale)