chore: update down.sql of ext_data

frontend/rust-lib/flowy-database/migrations/2022-06-11-090029_view-add-col/down.sql

@@ -1 +1,2 @@
--- This file should undo anything in `up.sql`
+-- This file should undo anything in `up.sql`
+ALTER TABLE view_table DROP COLUMN ext_data;