Revert "feat(topics): add the Topics preview area, its navigation entry and its switch"

This reverts commit c68e833dcc.

src/hooks/use-preview-features.ts

@@ -1,14 +0,0 @@
-"use client";
-
-import { useAuth } from "@/hooks/use-auth";
-import type { PreviewFeatures } from "@/lib/settings/preview-features";
-
-/**
- * The person's own preview opt-ins, off the cached account payload, or null
- * while it has not answered. Every reader treats null as "off": a preview
- * appears only once the server has said yes, never on a guess.
- */
-export function usePreviewFeatures(): PreviewFeatures | null {
-  const { user } = useAuth();
-  return user?.previewFeatures ?? null;
-}