add missing migration to drop websub table

priv/repo/migrations/20191017225002_drop_websub_tables.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.DropWebsubTables do
+  use Ecto.Migration
+
+  def change do
+    drop_if_exists(table(:websub_client_subscriptions))
+    drop_if_exists(table(:websub_server_subscriptions))
+  end
+end