These steps will remove all of the previously synced photos from the device.
To delete photos that have been synced from a computer that's no longer accessible:
- Create a folder on the computer that you now sync with, and add a single photo to that folder.
- In the Photos tab in iTunes, click the box next to "Sync Photos From."
- In the pop-up menu next to "Sync Photos From," choose the folder you created in step 1.
- Apply the change.
- Uncheck "Sync Photos From."
- Apply the change again.
- Your synced photos will now be removed from the device.