Remove StyleSheetScopingNodeList

After <style scoped> removal (crbug.com/355674),
StyleSheetScopingNodeList is no longer necessary and can be deleted.

This CL removes StyleSheetScopingNodeList.{h,cpp} and
relevant pieces.

BUG=393905
TEST=no layouttest fails

Review URL: https://codereview.chromium.org/390343002

git-svn-id: svn://svn.chromium.org/blink/trunk@178288 bbb929c8-8fbe-4397-9dbb-9b2b20218538