Remove Node.prefix so it's no longer visible to script.

This is in line with DOM4 <http://dom.spec.whatwg.org/#node>

prefix remains on Attr and Element, which is the only place it does
something useful, although setting prefixes will probably be removed
entirely.

BUG=325922

Review URL: https://codereview.chromium.org/126713004

git-svn-id: svn://svn.chromium.org/blink/trunk@164729 bbb929c8-8fbe-4397-9dbb-9b2b20218538