Remove deprecated method Notification::detachPresenter()

In Chromium, the only implementation of WebNotificationPresenter,
content::NotificationProvider never goes away. Rather than implementing it or
leaving it empty, just remove it.

BUG=none

Review URL: https://codereview.chromium.org/36823002

git-svn-id: svn://svn.chromium.org/blink/trunk@161539 bbb929c8-8fbe-4397-9dbb-9b2b20218538