How can I dismiss the App Store banner in Notification Center without taking an action?
To dismiss the notification temporarily: 

Click and hold the banner and flick it to the right to put that notification in Notification Center. If you are using a Trackpad, You can hover over the alert and use two fingers to flick it to the left.

The notification will still remain in the Notification Center until you update.

Unfortunately, there is currently no way to dismiss the notification permanently.