refactor: remove stale LandingPage entry from storybook migration list

The LandingPage feature and its stories were entirely deleted in 6241d6d.
The MIGRATED_APP_STORIES entry that guarded against re-creating the old
App.landingPage.stories.tsx in src/browser/stories/ is now dead config —
there is no colocated version to protect anymore.

Author: mux-bot[bot]

tests/ui/storybook/coverage.test.ts

@@ -24,7 +24,6 @@ const REQUIRED_COLOCATED_STORIES = [
 const MIGRATED_APP_STORIES = [
   "App.sidebar.stories.tsx",
   "App.welcome.stories.tsx",
-  "App.landingPage.stories.tsx",
   "App.errors.stories.tsx",
   "App.titlebar.stories.tsx",
   "App.projectCreate.stories.tsx",