[css-sizing] Revert 45d4464, it violates the resolution from #2708

[Loirooriol]

#2708 had a clear resolution: intrinsic keyword values in the block axis should just behave as auto, but not compute to auto. The spec was changed in b603bfb.

Later, #3973 asked to obey the intrinsic keyword values instead of making them behave as auto in the block axis. 45d4464 was supposed to do this change but:

• It just made intrinsic keywords compute back to auto, basically reverting b603bfb, against the resolution from [css-sizing] Incorrect use of the word "compute" in definition of intrinsic keyword values #2708
• It didn't address [css-sizing-3] Why was min-content, etc. redefined to do nothing in the block axis? #3973 either, since computing to auto implies behaving as auto.
• The changes did the opposite of what the commit message said

Please revert it. CC @tabatkins, @fantasai

[fantasai]

Deferring discussion back to #3973.