Delete unnecessary language from 'Derived Work' definition.

As noted by pchestek: "such that copyright permission is required"
"doesn't add anything. A Derived Work by definition requires
permission to create; if it didn't it's not an infringement and the
license doesn't need to be invoked."

Though pchestek follows that with this note: "I agree, though, that it
seems clearer with the extra wording so it might be worth the
verbosity sacrifice."
See:
https://github.com/richardfontana/copyleft-next/pull/19