fix: remove unmaintained Mastodon link from footer

The Mastodon account @nodejs on social.lfx.dev is no longer maintained
by the foundation's marketing team per community discussion.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

Author: 7487

apps/site/navigation.json

@@ -84,11 +84,6 @@
       "link": "https://discord.gg/nodejs",
       "alt": "Discord"
     },
-    {
-      "icon": "mastodon",
-      "link": "https://social.lfx.dev/@nodejs",
-      "alt": "Mastodon"
-    },
     {
       "icon": "bluesky",
       "link": "https://bsky.app/profile/nodejs.org",