Remove plainDateTimeOrRelativeToWillBeUsed

[ptomato]

From #2758 reviews. This can likely be drastically simplified after #2826 lands and can maybe even be done as part of that issue.

This plainDateTimeOrRelativeToWillBeUsed is a little hard to reason about. Here's how you can make it clearer. Feel free to queue this up as an Editorial task for later if the goal here is keep the diff as small as possible.

You can determine whether nanosecond-rounding is possible at the beginning of the function and it makes things clearer. Pseudocode:

const needRelativeRounding =
  zonedRelativeTo || // if zoned date time
  // OR, if plain date and there's a unit >DAY
  ES.IsCalendarUnit(largestUnit) ||
  calendarUnitsPresent

if (!needRelativeRounding) {
  // do nanosecond-based rounding (BalanceTimeDuration)
  // return the result here, short circuiting
}

if (!zonedRelativeTo || !plainRelativeTo) {
  throw new RangeError('need a relativeTo')
}

const calendarRec = CalendarMethodRecord.CreateFromRelativeTo(plainRelativeTo, zonedRelativeTo, [
  'dateAdd',
  'dateUntil'
]);

if (zonedRelativeTo) {
  const precalculatedPlainDateTime = // ...
  // proceed with DifferenceZonedDateTimeWithRounding...
} else if (plainRelativeTo) {
  // proceed with DifferencePlainDateTimeWithRounding...
}

Originally posted by @arshaw in #2758 (comment)