Clean up the ash switch ash-disable-usb-charger-notification.

BUG=348787
R=mukai@chromium.org

Review URL: https://codereview.chromium.org/181063023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254800 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254800