Delete all photos from iphone

See the following from Get help with iCloud Photos - Apple Support

(Note that they will delete everywhere)

How do I remove all iCloud Photos content from iCloud and my devices?

To turn off your iCloud Photos everywhere, follow these steps:

• On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Manage Storage > Photos, then tap Disable and Delete. 
• On your Mac, go to Apple menu > System Preferences > iCloud. In the lower-right corner, click Manage, select iCloud Photos,* then select Disable and delete.

Photos and videos are stored in your account for 30 days. To download your photos and videos on your iOS device, go to Settings > [your name] > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. You can also select the photos and videos that you want to download from iCloud.com.