chore: remove Letterspace newsletter integration and all LETTERSPACE_* variables

packages/frontend/src/routes/magazine/[slug]/+page.svelte

@@ -9,7 +9,6 @@ import { getAssetUrl } from "$lib/directus";
 import SharingPopup from "$lib/components/sharing-popup/sharing-popup.svelte";
 import Meta from "$lib/components/meta/meta.svelte";
 import PeonyBackground from "$lib/components/background/peony-background.svelte";
-import NewsletterSignup from "$lib/components/newsletter-signup/newsletter-signup-widget.svelte";
 import SharingPopupButton from "$lib/components/sharing-popup/sharing-popup-button.svelte";
 
 const { data } = $props();
@@ -215,8 +214,6 @@ const timeAgo = new TimeAgo("en");
         </Card>
         -->
 
-                <!-- <NewsletterSignup email={data.authStatus.user?.email}/> -->
-
                 <!-- Back to Magazine -->
                 <Button
                     variant="outline"