tidy(migrator): remove logic to check if graph_executions exists in migration 5

Initially I wanted to show how many sessions were being deleted. In hindsight, this is not great:
- It requires extra logic in the migrator, which should be as simple as possible.
- It may be alarming to see "Clearing 224591 old sessions".

The app still reports on freed space during the DB startup logic.

invokeai/app/services/shared/sqlite/sqlite_util.py

@@ -32,7 +32,7 @@ def init_db(config: InvokeAIAppConfig, logger: Logger, image_files: ImageFileSto
     migrator.register_migration(build_migration_2(image_files=image_files, logger=logger))
     migrator.register_migration(build_migration_3(app_config=config, logger=logger))
     migrator.register_migration(build_migration_4())
-    migrator.register_migration(build_migration_5(logger=logger))
+    migrator.register_migration(build_migration_5())
     migrator.run_migrations()
 
     return db