feat(admin): simplify phrase generation by removing missing phrases endpoint

python/ebook_search/api/templates/admin.html

@@ -60,13 +60,6 @@ head %}
     >
       <button type="submit">Regenerate all phrases</button>
     </form>
-    <form
-      hx-post="/admin/phrases/generate-missing"
-      hx-target="#admin-status"
-      hx-swap="innerHTML"
-    >
-      <button type="submit">Add missing phrases</button>
-    </form>
     <form
       hx-post="/admin/phrases/judge-all"
       hx-target="#admin-status"