Remove unused feature ElementSetAttributeNodeNS

Intent to Deprecate and Remove:
    https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/jS7iCEmoWfQ

BUG=341296

Review URL: https://codereview.chromium.org/143173003

git-svn-id: svn://svn.chromium.org/blink/trunk@166699 bbb929c8-8fbe-4397-9dbb-9b2b20218538