removeObserver(observer) {
        const observers = this.observers_;
        const index = observers.indexOf(observer);

        // Remove observer if it's present in registry.
        if (~index) {
            observers.splice(index, 1);
        }

        // Remove listeners if controller has no connected observers.
        if (!observers.length && this.connected_) {
            this.disconnect_();
        }
    }
Removes observer from observers list.

@param {ResizeObserverSPI} observer - Observer to be removed.
@returns {void}