Remove the last remnant of <style scoped>

This code was added while <style scoped> was still supported:
http://trac.webkit.org/changeset/125955

The scoped attribute now does nothing anywhere else, so special behavior
for editing doesn't make sense.

TEST=LayoutTests/editing/execCommand/delete-selection-has-style.html

BUG=379096

Review URL: https://codereview.chromium.org/433663002

git-svn-id: svn://svn.chromium.org/blink/trunk@179498 bbb929c8-8fbe-4397-9dbb-9b2b20218538