I would first try starting Eclipse with a brand new workspace, and then see if it will start. It might start because the SlickEdit plugin might not try to activate automatically unless you do something to trigger it to activate. If Eclipse will start then you'll be able to uninstall via the normal Eclipse installation mechanism.
If you can't get that to work, I would suggest manually deleting all the com.slickedit.* directories under the ECLIPSEDIR/plugins and ECLIPSEDIR/features directories, and then starting Eclipse.
- Ryan