Remove WebNode::remove().

The only user was inside blink in WebFrameTest and we can just use the
real Node::remove() there.

Review URL: https://codereview.chromium.org/1343403004

git-svn-id: svn://svn.chromium.org/blink/trunk@202467 bbb929c8-8fbe-4397-9dbb-9b2b20218538