Packed charts can be found in the Downloads page. If you want to delete items that were downloaded using Pack, you can do so by going to MORE > DOWNLOADS and swiping from right-to-left across the item you want to delete, then tapping the DELETE button that appears.
Image 1: Swipe from right to left across an item to reveal a delete button.
Beginning with ForeFlight Mobile version 7.1, packed charts that do not belong to any of the states that are "subscribed" to in Download Settings are stored in an expandable group called "Packed and Unselected Regions". Both the entire group and individual items within it can be deleted using the swipe-left motion.
Image 2: Packed items are stored in "Packed and Unselected Regions" in ForeFlight Mobile 7.1.