joplin_desktop: drop panel_admin (admin-only is the default; linter rejects it)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

joplin_desktop/config.yaml

@@ -68,7 +68,6 @@ options:
   data_location: /config/data
   keyfile: privkey.pem
   use_own_certs: false
-panel_admin: true
 panel_icon: mdi:notebook-edit
 ports:
   3000/tcp: null