Remove stale libressl-4.0.0 entry from prune matrix (#129)

The builder was dropped in a prior cycle but its image name was left
behind in the prune-untagged-builder-images matrix. No builder folder
or CI job references it.

Closes #121

Author: SeanTAllen

.github/workflows/rebuild-ponyc-based-images.yml

@@ -813,7 +813,6 @@ jobs:
       matrix:
         image:
           - shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-3.9.2
-          - shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-4.0.0
           - shared-docker-ci-x86-64-unknown-linux-builder-with-openssl_1.1.1w
 
     steps: