UIStackView : Is it really necessary to call both removeFromSuperView and removeArrangedSubview to remove a subview?

No, just call subview.removeFromSuperview()

/* Removes a subview from the list of arranged subviews without removing it as
 a subview of the receiver.
    To remove the view as a subview, send it -removeFromSuperview as usual;
 the relevant UIStackView will remove it from its arrangedSubviews list
 automatically.
 */
open func removeArrangedSubview(_ view: UIView)