db: drop tag subscriptions table.

db/migrate/20191117074642_drop_tag_subscriptions.rb

@@ -0,0 +1,7 @@
+require_relative "20100307073438_create_tag_subscriptions"
+
+class DropTagSubscriptions < ActiveRecord::Migration[6.0]
+  def change
+    revert CreateTagSubscriptions
+  end
+end