Remove VoxelPrimitive.jitter option

[jjhembd]

The VoxelPrimitive.jitter option applies random shifts to the starting point for voxel ray-marching. This helped blur artifacts at the edges of voxels when using a constant step size.

After #11875, voxels use an optimum step size which fixes the artifacts. This makes the jitter unnecessary, and also effectively ignores the jitter after the first step.

This option should be removed. Since the voxel API is flagged experimental, we may not need a deprecation period.