How to force empty Trash if it has system files?

The question title may seem a duplicate, but this case of mine is unique. This happened because of this problem, which I was able to answer/solve. https://superuser.com/questions/1447825/will-re-installing-mojave-through-internet-recovery-delete-data

And now the system files of the Previous Content, from the previous installed OS cannot be deleted. It seems the MacOS sees these files as legitimate system files, while in fact, they're not.

I'm using Mojave now btw.

Try the following:

• Open the Trash, click on it in the Dock.

• Open Terminal and type the following command, followed by a space:

   sudo rm -rf 
  

• Now drag and drop the target folder from the Trash onto the Terminal window.

• Press Enter and type in your (Admin) password and press Enter.

If that doesn't remove the target folder, then make note of its pathname, writing it down on a piece of paper.

• Reboot the Mac to the Recovery HD, pressing ⌘R, and holding it, as the computer starts up after having shut it down.
• Once the macOS Utilities window appears, select Terminal from the Utilities menu.

• In Terminal type the following command:

   rm -rf /path/to/target/folder
  

Example:

rm -rf /Volumes/Macintosh\ HD/Users/johndoe/.Trash/Previous\ Content/Devices

• Note: Because this is being done from the Recovery HD, you need to add e.g. /Volumes/Macintosh\ HD to the pathname you wrote down earlier.

NOTE: Make sure you type the fully qualified pathname correctly as once you press Enter, that's it, it's gone and cannot be recovered!

• Then type reboot and press Enter.

See: About macOS Recovery