Remove GN GYP generator.

This removes the GYP generator code and associated build stuff.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/206813002

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@259983 4ff67af0-8c30-449e-8e8b-ad334ec8d88c