chore: remove stale silverblue-main entry from image-versions.yml (#26)

PR #17 moved the base image from ghcr.io/ublue-os/silverblue-main to
quay.io/fedora-ostree-desktops/silverblue. The Justfile only reads
'common' and 'brew' entries from image-versions.yml, so the
silverblue-main entry is dead config that may cause unnecessary
Renovate PRs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: castrojo

image-versions.yml

@@ -1,8 +1,4 @@
 images:
-  - name: silverblue-main
-    image: ghcr.io/ublue-os/silverblue-main
-    tag: latest
-    digest: sha256:7a0a6155e368390fe8acc583565cdb38849422960183724aa5fdba950a5e536e
   - name: common
     image: ghcr.io/projectbluefin/common
     tag: latest