how do you delete photos from iPad when the original iTunes used to transfer is not available

Not sure what you mean by the "original iTunes on the computer used" but the information below is from Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support

Note that if the photos are no longer on the computer then you would use the link in the last line below.

Delete synced photos

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.

It will work if you follow the instructions as I noted in my reply above. If the photos are no longer on a computer then you need to follow the instructions in the last line as noted in my post. I will copy them and show them below. The photos you are trying to delete won't be visible; you will need to change the settings on the Photos tab of iTunes to select a new photo as shown below, that will sync the one new photo to your phone and delete all other previously synced photos, then you sync again to delete the one new photo.

Once they are deleted, if you have other photos on your Mac that you still want to be synced to the phone then you will change the settings on the Photos tab of iTunes to select them and sync one more time to add them back to the phone.

Delete photos and video synced from iTunes when you can’t access the originals

You can delete photos you synced from iTunes by syncing them with a new folder.

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:

1. Create a folder on the computer that you now sync with, and add a single photo to that folder.
2. In the Photos tab in iTunes, click the box next to "Sync Photos From."
3. In the pop-up menu next to "Sync Photos From," choose the folder you created in step 1.
4. Apply the change.
5. Uncheck "Sync Photos From."
6. Apply the change again.
7. Your synced photos will now be removed from the device.