Consider removing `ToQuaternion`

[gkjohnson]

ToQuaternion basically only works with properly with Unity-frame axes because we have to assume certain conventions when pulling the new rotation order out of the axis.

There even seem to be subtle rotation offsets when consistently converting to a different frame like XYZ instead of XY-Z. Possibly because the euler application for quaternions assumes the axis conventions? This should be investigated (the problem can be seen when rotating like in the example image)

Either way, it might make things less confusing if it wasn't there