I just found out another way to delete all the photos at once on iPhone while playing around with my device. No need to use Mac or PC.
I used a feature in iOS 9 that is often overlooked by many users. This feature allows us to select multiple photos just by performing a swipe of a finger.
This is how the feature works:
1. Open the Photos app, go to an album
2.Tap on the Select button.
3.Tap and drag from one photo through the range of photos that you wish to select. As you drag your finger, the blue checkmark icon will appear on each thumbnail, indicating that the photo is selected.
Now we'll use it to select all the photos and delete them:
1. Tap and hold on the photo at the bottom (the most recent one) to select it. Then slide in any direction to select one or two more.
2. Without lifting your finger, use another finger to tap the status bar. Now you’ll be taken to the top of the album, keep sliding to select the rest.
3. Ok now you can hit the Trash button to delete everything.