If you don't boot from it, can't you just delete the system folder?
If it gives you "not permitted" or something like that, try clicking on the 10.3 system folder, chose "Get info", then change the "ownership & permissions" to you instead of system, apply to enclosed items, and then delete the folder.
If that doesn't work, I believe you can use Terminal, navigate to where your 10.3 system folder is located, and type: "rm system". But you really want to check up on that.