Disable 2020 Attendee Integration

[froboy]

Problem/Motivation
An old integration is bringing bogus numbered Attendee node into the site.

Steps to reproduce
Visit https://www.midcamp.org/admin/content, observe lots of attendee nodes without any info other than a number.

Proposed resolution
Disable this integration and delete the junk nodes.

Duplicated at https://www.drupal.org/project/midcamp/issues/3195752