If you just want to delete the demo package, delete every file starting with demo.* in these folders:
https://github.com/openhab/openhab-distro/tree/master/features/distro-resources/src/main/resources
If you want a complete clean start delete also your JsonDB files. Note: everything is gone then, not just the demo package.
Always do a backup first!
It is probably easier to just do a clean openHAB install from scratch.
Special greetings to my neighbour town in East Frisia 