docs: remove migration comments from database schema

- Removed comment about user_ebook_folders table replacement
- Removed comment about library system transition
- Historical migration documentation removed

This is part of legacy code cleanup Phase 1.
Phase 1: Documentation Cleanup

database/schema/schema.sql

@@ -367,9 +367,6 @@ JOIN libraries l ON mi.library_id = l.id
 JOIN library_types lt ON l.library_type_id = lt.id
 WHERE lt.name = 'ebooks';
 
--- Note: user_ebook_folders table is replaced by library_folders table
--- Libraries now handle folder management instead of individual users
-
 -- Create indexes for better query performance
 CREATE INDEX idx_users_email ON users(email);
 CREATE INDEX idx_users_username ON users(username);