Remove EditableLinkBehavior, which is always set to NeverLive by Chromium.

On the Chromium side, there's a comment that this is unlikely to ever change.
And indeed it has never changed since the initial commit of webkit/ in 2008.

Thus this code is dead in Blink and should be removed.

Chromium CL: https://codereview.chromium.org/236163005/

BUG=362961
TEST=ManualTests/contenteditable-link.html

Review URL: https://codereview.chromium.org/236203005

git-svn-id: svn://svn.chromium.org/blink/trunk@171412 bbb929c8-8fbe-4397-9dbb-9b2b20218538