fix: remove MDC migration guide

Removes the MDC migration guide since we've removed the migration in v18.

Author: crisbeto

src/app/shared/guide-items/guide-items.ts

@@ -74,12 +74,6 @@ const GUIDES = [
     document: '/docs-content/guides/duplicate-theming-styles.html',
     overview: 'Learn about our new color mixins for Sass that avoid duplicating theming styles.'
   },
-  {
-    id: 'mdc-migration',
-    name: 'Migrating to MDC-based Components',
-    document: '/docs-content/guides/v15-mdc-migration.html',
-    overview: 'Learn how to migrate your app to the new MDC-based components available in v15.'
-  },
   {
     id: 'material-2-theming',
     name: 'Theming Angular Material with Material 2',