Removes all 3D modification resources. SkeletonIK3D is a node and still supported.
Remove deprecated Skeleton3D functionality for 4.0, so we can add it back in 4.x.
Remove local_pose_override feature from Skeleton3D and BoneAttachment3D.
Expose Skeleton3D::get_version() so IK scripts/extensions can cache bones.
Note: This change only affects 3D. SkeletonModification2D will work as before.