AddDuration: Unnecessary call to GetPlainDateTimeFor

[anba]

AddDuration only needs to call GetPlainDateTimeFor when calendar units are present. Current steps 10-11 can be rewritten as follows:

10. If precalculatedPlainDateTime is undefined and largestUnit is one of "year", "month", "week", or "day", then
    1. Let startDateTime be ? GetPlainDateTimeFor(timeZone, zonedRelativeTo.[[Nanoseconds]], calendar).
11. Else,
    1. Let startDateTime be precalculatedPlainDateTime.

When largestUnit is smaller than "day", all calendar units ({y,mon,w,d}{1,2}) are zero, so the two AddZonedDateTime calls will take the AddInstant fast path and no Temporal.PlainDateTime object is needed.

[ptomato]

Thanks for spotting, as usual! I'll add a fix for this one on to part 4 (#2519).