Drop pacman packaging, bakery-only distribution

bakery already fully covers what the PKGBUILD provided (binary, systemd
--user service where applicable, dependency declarations) except a LICENSE
copy, which bakery.toml's new license_file field now closes. Removes
packaging/arch/ and .forgejo/workflows/package.yml; adds the LICENSE
artifact to each release/dev-release/beta-release workflow's prepare
step. Not pacman-installed inside BOS today (BOS already consumes these
apps exclusively via build-local.sh's skel-staging), so this only removes
the option to `pacman -S` outside of BOS/bakery.

.forgejo/workflows/beta-release.yml

@@ -56,6 +56,7 @@ jobs:
               > "${PKG_DIR}/${bin}-x86_64.sha256"
           done
           cp src/breadpad.example.toml "${PKG_DIR}/"
+          cp src/LICENSE "${PKG_DIR}/"
           cp src/bakery.toml "${PKG_DIR}/bakery.toml"
           ln -sfn "${VERSION}" "/srv/breadway-dl/beta/breadpad/latest"