message: Remove an unused forward declaration
diff: @@ -38,7 +38,6 @@ class SyncUser;
  namespace _impl {
  class RealmCoordinator;
  struct SyncClient;
  -class WriteTransactionNotifyingSync;
  namespace sync_session_states {
  struct Active;
  @@ -251,7 +250,6 @@ public:
  // without making it public to everyone
  class Internal {
  friend class _impl::RealmCoordinator;
  - friend class _impl::WriteTransactionNotifyingSync;
  static void set_sync_transact_callback(SyncSession& session,
  std::function<SyncSessionTransactCallback> callback)
  