UX: hide topic layout btn on Horizon (#37847)

This button is not hooked up in any way to the Horizon topic cards.
Hiding for now, until we repurpose for low-context/high-context
switching

themes/horizon/scss/hiddenstuff.scss

@@ -3,7 +3,8 @@
 .sidebar__panel-switch-button,
 .notifications-button-footer .reason .text,
 .pinned-button .reason .text,
-.more-topics__browse-more {
+.more-topics__browse-more,
+.topic-list-layout-trigger {
   display: none;
 }