How to disable recycle bin (Recently Deleted) of iOS 8 Photos app?
There is one way I know of, if the device is jailbroken.  It's a free tweak on the "BigBoss" Cydia Repo (http://apt.thebigboss.org/repofiles/cydia/), called "NoRecentlyDeleted".  

If your device is jailbroken, just search for "NoRecentlyDeleted" on Cydia, and install it.  After you install and respring your device, the photos you delete will no longer go into the "Recently Deleted" album in your photos app, and will instead be deleted from the device, as they were previously in iOS.  

As of now, the tweak does not fully remove the "Recently Deleted" items album in the photos app, and it will still show there.  The author of the tweak plans on updating it in the near future, so the album is removed as well.

I hope this helps ;)  

-James