removeEventListener(type, fn) {
            if (!typeof type === "string") {
                throw new TypeError(`Expected string, got ${typeof type}`);
            }
            if (typeof fn !== "function") {
                throw new TypeError(`Expected function, got ${typeof fn}`);
            }
            let listeners = this.eventListeners[type];
            if (listeners) {
                listeners.delete(fn);
            }
        }
Remove an event callback function.

@param {string} type - Event type.
@param {Function} fn - Callback function to remove.