refactor: drop the Alchementary Link data migration

Startup no longer looks for an AlchementaryLink data directory to copy
into NexusTerminal, and the 'Imported your Alchementary Link data' toast
and the migration docs are gone. Existing Alchementary Link installs are
no longer picked up automatically.

rust/app/src/app.rs

@@ -210,9 +210,6 @@ impl App {
             *sid = Some(id.clone());
             app.add_session(&id, true);
         }
-        if let Some(from) = &svc.config.migrated_from {
-            app.s.success(format!("Imported your Alchementary Link data from {}", from.display()));
-        }
         // Background update check, once per launch.
         if app.screenshot.is_none() && app.s.prefs.check_for_updates {
             let svc2 = svc.clone();