After you uninstall Avast,
You could safely remove Avast devices drivers from the Device Manager. (Make sure you check View → Show Hidden Devices.)
After you uninstall those devices, those registry entries will be deleted.

LEGACY registry entries could only be accessed by the SYSTEM. If you are an administrator, you could set Full Access for yourself. But I suggest you using the above method instead.

Make sure Avast is fully uninstalled before performing this action.