Remove unnecessary SSR prefetch for user subscriptions

The SSR prefetch for getSubscriptions(observer) was wasteful:

- For non-logged-in users: Fetched hive.blog's subscriptions (DEFAULT_OBSERVER)
  which were never used since the client-side query is disabled
- For logged-in users: Client fetches again anyway after hydration

The subscriptions data is used for the "My Communities" sidebar which:
- Is not SEO-critical (bots don't need personal subscriptions)
- Is not above-the-fold critical (sidebar content)
- Is personal data only relevant when logged in

The client-side query in main-page-layout.tsx already handles this correctly:
- Fetches only when user.isLoggedIn
- Shows CommunitiesSidebar (generic list) as fallback

This saves 1 API call per feed page for all users.

Fixes #790