Retire the elephant packages and omarchy-walker (#872)

Omarchy 4 no longer uses Elephant or the omarchy-walker meta package, and
omarchy-upgrade-to-quattro removes them; every channel serves Omarchy 4.0.4.
Nothing else here depends on them. walker itself stays.

Deleting a recipe stops it building but leaves it in the channel databases,
and the only removal tool worked on the old repository host's local tree.
Add bin/unpublish-packages, publish-artifact's counterpart: pull the channel
database, repo-remove every entry built from the named pkgbases, upload it.
Package files stay in the bucket. unpublish.yml runs it per channel and
architecture under the publish lock, for packages with no recipe on master.

bin/build

@@ -86,7 +86,7 @@ while [[ $# -gt 0 ]]; do
     echo "  $0 --arch aarch64"
     echo "  $0 --mirror stable"
     echo "  $0 --package yay"
-    echo "  $0 --package yay elephant cursor-bin"
+    echo "  $0 --package yay walker cursor-bin"
     echo ""
     echo "Environment (for CI and resumed builds; defaults keep today's behaviour):"
     echo "  OMARCHY_KEEP_BUILD_WORKSPACE=1   keep build-output and reuse packages already there"