Apple has a good knowledge base article, HT4236, that explains how to sync and/or remove photos from your iPhone. These are the instructions that they supply for removing synced ("My Computer") photos from your iPhone:

1. In iTunes, click the Device button in the upper right corner. (If viewing the iTunes Store, click the Library button in the upper-right corner first.)
2. Click the Photos tab in the resulting window.
3. Choose "Selected albums" and deselect the albums or collections you want to delete. Note: To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Alternately, if you're trading in or selling the iPhone I would recommend that you wipe the data off completely, not just the photos. You can do this from the iPhone:

1. Open Settings
2. Tap on General
3. Scroll to the bottom and tap on Reset
4. Tap on "Erase All Content and Settings"
5. Enter your iPhone's lock code and tap OK to erase everything

Once it's finished, usually after a few minutes, it will be just as it was from the factory.