LengthOrAutoOrCalculated::without_auto()
...when `LengthOrAutoOrCalculated` holds calculated value. We were incorrectly assuming that if contained value is not auto, then it must be a length. Fixes crashing on https://hollowknightsilksong.com/
This appears to have been a bug in the spec which was later corrected - so to fix the crash we can simply remove this assertion. Fixes: #868