Drop old readonly discourse_calendar tables.

It has been 2 weeks since the tables were migrated. Safe to drop.

This reverts commit c17bbf57d1.

db/post_migrate/20200805073343_drop_old_discourse_calendar_tables.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'migration/table_dropper'
+
+class DropOldDiscourseCalendarTables < ActiveRecord::Migration[6.0]
+  def up
+    if table_exists?(:discourse_calendar_post_events)
+      Migration::TableDropper.execute_drop(:discourse_calendar_post_events)
+    end
+
+    if table_exists?(:discourse_calendar_invitees)
+      Migration::TableDropper.execute_drop(:discourse_calendar_invitees)
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrrelversibleMigration
+  end
+end