Remove "_scope" prop from User class

[mStirner]

module.exports = class User extends Item {

    constructor(scope, obj) {

        super(obj);

        // removed for #356
        //Object.assign(this, obj);
        //this._id = String(obj._id);

        Object.defineProperty(this, "_scope", {
            value: scope,
            writable: false,
            enumerable: false,
            configurable: false
        });

    }
...