message: PanelHeaderContent: remove empty rulesets
diff: /*
  iOS
  */
  -.PanelHeaderContent--ios {
  - }
  -
  .PanelHeaderContent--ios .PanelHeaderContent__in.Tappable--active {
  opacity: .7;
  }
  /*
  Android
  */
  -.PanelHeaderContent--android {
  - }
  -
  .PanelHeaderContent--android .PanelHeaderContent__in.Tappable--active,
  .PanelHeaderContent--vkcom .PanelHeaderContent__in.Tappable--active {
  background-color: rgba(255, 255, 255, .1);
  