ci: remove github actions preview pipeline (ref denog/routing-guide#116)

During denog member days we decided to use an external pipeline for previews,
to enable previews of forks.

Running github actions, which we use in production to deploy this site, for
forks, may cause security issues, which we avoid by using e. g. netlify.

Author: felbinger

.github/workflows/preview.yml

@@ -57,7 +57,8 @@ markdown_extensions:
           class: mermaid
           format: !!python/name:pymdownx.superfences.fence_code_format
 plugins:
-  - git-revision-date-localized
+  - git-revision-date-localized:
+      enable_parallel_processing: false
   - tags
   - search
 extra: