If class A is derived from class B, then an instance of class A can be
converted to B via a static cast, so a slower QI is not needed.
Differential Revision: https://phabricator.services.mozilla.com/D6861
--HG--
extra : moz-landing-system : lando
This API is deprecated (callers are supposed to be using
nsISound::playEventSound instead) and there are no callers remaining in
mozilla-central, or references to the strings for the API. Let's remove
dead code.