Remove outdated reference to icon_installing.png

void WearRecoveryUI::Init() {

    ScreenRecoveryUI::Init();

    LoadBitmap("icon_installing", &backgroundIcon[INSTALLING_UPDATE]);

    backgroundIcon[ERASING] = backgroundIcon[INSTALLING_UPDATE];

    LoadBitmap("icon_error", &backgroundIcon[ERROR]);

    backgroundIcon[NO_COMMAND] = backgroundIcon[ERROR];

    // This leaves backgroundIcon[INSTALLING_UPDATE] and backgroundIcon[ERASING]

    // as NULL which is fine since draw_background_locked() doesn't use them.

}

void WearRecoveryUI::SetStage(int current, int max)