remove obsolete interface WebKitPlatformSupport::prefetchHostName.

The use of this API was removed in Blink@r153315. The chromium side of this patch is http://crrev.com/21043002

BUG=None

Review URL: https://chromiumcodereview.appspot.com/20992004

git-svn-id: svn://svn.chromium.org/blink/trunk@155136 bbb929c8-8fbe-4397-9dbb-9b2b20218538