Q: How do I delete photos from my iPhone that were previously synced FROM my PC?

You have to do it through iTunes. Open iTunes and uncheck the sync photos option. The photos will be removed.

Have you tried all of the following in the quote below from Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support

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.