chore: remove classy theme too

[lazysoundsystem]

Refs: OPS-11317

Further to #1004 and not properly tested, but I don't think Classy is being used anywhere - and uninstalling it doesn't seem to make any difference. It was removed from SLT when we upgraded to Drupal9 and also isn't yet Drupal 11 compatible: https://www.drupal.org/project/classy/issues/3461089

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Pusher: @lazysoundsystem, Action: pull_request, Workflow: Run tests