upgraded adobe programs do not remove previous versions.
you can uninstall pse 8 if you do not need it, but you do not need to uninstall it. occasionally there are good reasons to not uninstall so you should probably use your pse13 for one or two months to see if you ever need your pse8. if not, that would be a good time to uninstall it.
just make sure you maintain a record of your pse8 serial number if pse13 was an upgrade because you'll need that in the future if you want install pse13 on another (or the same) computer and pse8 is not installed.