chore(ci): drop the unreachable PR image tag

The merge job asked the metadata action for a pull request tag. The
workflow starts on push only, so a pull request event never arrives and
the rule never applies.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: ledermann

.github/workflows/ci.yml

@@ -311,7 +311,6 @@ jobs:
         with:
           images: ghcr.io/solectrus/helios
           tags: |
-            type=ref,event=pr
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}