Undo unintentional comment-out and tweak formatting.
Restore `up_axis` to the original `Z_UP`.

These were changed from `Z_UP` to `Y_UP` to `Z` during the axis
excursions. three.js' Collada parser only looks at the first
character, so the behavior should be the same. But we shouldn't keep a
change for no reason.

References #1680.
Remove unneeded backup.

References #1680.