What is this? UIView's border line or shadow? And how to remove it from the background?

Question

I'm creating a dynamic animation system(UIKit Dynamics) with an object orbiting around a point (the center of the orbit). Radial gravity and Vortex, 2 simultaneous forces make the blue object orbit around the yellow object. They are UIFieldBehavior both.

lazy var animator: UIDynamicAnimator = {
    return UIDynamicAnimator(referenceView: self.view)
}()

lazy var center: CGPoint = {
    return CGPoint(x: self.view.frame.midX, y: self.view.frame.midY)
}()

lazy var radialGravity: UIFieldBehavior = {
    let radialGravity: UIFieldBehavior = UIFieldBehavior.radialGravityField(position: self.center)
    radialGravity.region = UIRegion(radius: 300.0)
    radialGravity.strength = 1.5
    radialGravity.falloff = 4.0
    radialGravity.minimumRadius = 50.0
    return radialGravity
}()

lazy var vortex: UIFieldBehavior = {
    let vortex: UIFieldBehavior = UIFieldBehavior.vortexField()
    vortex.position = self.center
    vortex.region = UIRegion(radius: 200.0)
    vortex.strength = 0.005
    return vortex
}()

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    animator.setValue(true, forKey: "debugEnabled")
    let itemBehavior = UIDynamicItemBehavior(items: [orbitingView])
    itemBehavior.density = 0.5
    animator.addBehavior(itemBehavior)

    vortex.addItem(orbitingView)
    radialGravity.addItem(orbitingView)

    animator.addBehavior(radialGravity)
    animator.addBehavior(vortex)
}

I'm using the property observer to change the view layer properties and render a blue circle. And view's background blue border line is still there. How to remove it so there'll be only the circle?

@IBOutlet weak var orbitingView: UIView! {
    willSet {
        newValue.layer.cornerRadius = newValue.frame.width / 2.0
        newValue.layer.borderWidth = 0.0
        newValue.layer.backgroundColor = UIColor.blue.cgColor
    }
}