how do I get a legacy system extension off of my computer?

You do not need to do anything at this time. Apple is telling you that such extensions will not be supported in a futre version of macOS and that developers have to take care of it:

About legacy system extensions - Apple Support

I strongly recommend AGAINST removing legacy extensions today. Some new version of OSX will do that.

⚠️ The reason to leave this alone now is that some third-party legacy extensions are used by Apple as part of the current OS. Removing one now could create big issues with your computer.

Drag it to the Trash and authenticate.

Uninstall the software that put it there. Use the developer’s uninstalled or directions.