DifferenceISODateTime: All steps in DifferenceISODateTime are infallible

[anba]

No steps in DifferenceISODateTime should be fallible now that user-defined calendars are gone.

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Also:
Step 10 can be made clearer when using TemporalUnitCategory.

Instead of:

10. If largestUnit is not dateLargestUnit, then

Use:

10. If TemporalUnitCategory(largestUnit) is TIME, then

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Furthermore:
Step 7 can be simplified when swapping the inputs to CompareISODate:

Current:

5. Let dateSign be CompareISODate(isoDateTime2.[[ISODate]], isoDateTime1.[[ISODate]]).
   ...
6. If timeSign = -dateSign, then

Proposed:

5. Let dateSign be CompareISODate(isoDateTime1.[[ISODate]], isoDateTime2.[[ISODate]]).
   ...
6. If timeSign = dateSign, then