Your Honor, I want to recant my previous testimony....
The easiest and safest thing for you to do would be to boot to Recovery, open a Terminal, and use csrutil disableto turn off SIP. Then restart using the Apple menu, log on, open a Terminal and remove the unwanted files and directories. When done, you can boot back to Recovery, open a Terminal and use csrutil enableto turn SIP back on.
I'm changing my advice because after booting to Recovery you'll likely find that Macintosh HD - Data won't be mounted, and since that's where /Library actually lives you won't be able to delete anything from it. Mounting it isn't particularly difficult (details on request), but since hanging out in a Recovery mode Terminal can be an invitation to disaster, KISS is the best approach.