ao_coreaudio: do not signal per-app volume

CoreAudio doesn't seem to have this concept. The volume is reset the
next time audio is opened.

audio/out/ao_coreaudio.c

@@ -118,8 +118,6 @@ static int control(struct ao *ao, enum aocontrol cmd, void *arg)
         return set_volume(ao, arg);
     case AOCONTROL_HAS_SOFT_VOLUME:
         return CONTROL_TRUE;
-    case AOCONTROL_HAS_PER_APP_VOLUME:
-        return CONTROL_TRUE;
     }
     return CONTROL_UNKNOWN;
 }