The quickest way to delete all the photos from an iPad running ios 7 is to use the Image Capture program.
- Open Image Capture
- Connect you iPad to your Mac with USB cable
- Click on the iPad icon top left
- Select all by using ⌘A (or select manually if you want to selectively delete)
- Click the red circle symbol on the bottom and confirm you want to delete.
After a few seconds or minutes depending on the size they will be deleted from the iPad.