Editorial: Remove unnecessary CalendarDateFromFields from year-month …

…addition

Spec text algorithm corresponding to the previous commit. There is no need
to convert the end-of-month ISO date to a calendar date and then back to
an ISO date.

This looks like it might have removed an observable step, but I think
CalendarDateFromFields could already not throw here, because of the
assertion that _endOfMonth_ was a valid ISO date within range.

spec/plainyearmonth.html

@@ -759,10 +759,8 @@ <h1>
         1. If _sign_ &lt; 0, then
           1. Let _oneMonthDuration_ be ! CreateDateDurationRecord(0, 1, 0, 0).
           1. Let _nextMonth_ be ? CalendarDateAdd(_calendar_, _intermediateDate_, _oneMonthDuration_, *"constrain"*).
-          1. Let _endOfMonth_ be BalanceISODate(_nextMonth_.[[Year]], _nextMonth_.[[Month]], _nextMonth_.[[Day]] - 1).
-          1. Assert: ISODateWithinLimits(_endOfMonth_.[[Year]], _endOfMonth_.[[Month]], _endOfMonth_.[[Day]]) is *true*.
-          1. Set _fields_.[[Day]] to CalendarDay(_calendar_, _endOfMonth_).
-          1. Let _date_ be ? CalendarDateFromFields(_calendar_, _fields_, *"constrain"*).
+          1. Let _date_ be BalanceISODate(_nextMonth_.[[Year]], _nextMonth_.[[Month]], _nextMonth_.[[Day]] - 1).
+          1. Assert: ISODateWithinLimits(_date_.[[Year]], _date_.[[Month]], _date_.[[Day]]) is *true*.
         1. Else,
           1. Let _date_ be _intermediateDate_.
         1. Let _durationToAdd_ be ? CreateDateDurationRecord(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _days_).