removeSubscription(subscription: Object) {
    const eventType = subscription.eventType;
    const key = subscription.key;

    const subscriptionsForType = this._subscriptionsForType[eventType];
    if (subscriptionsForType) {
      delete subscriptionsForType[key];
    }
  }
Removes a specific subscription. Instead of calling this function, call
`subscription.remove()` directly.

@param {object} subscription