CI: remove redundant GITEA_RUNNER_LABELS from runner-compose.yml

Labels are defined in act-runner-config.yml which is the authoritative
source. The env var is only used on first registration and was being
ignored after .runner was created.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/runner-compose.yml

@@ -13,7 +13,6 @@ services:
       - GITEA_RUNNER_REGISTRATION_TOKEN=${RUNNER_TOKEN}
       - GITEA_RUNNER_NAME=swarm-runner
       # Maps the runs-on: ubuntu-latest label used in workflow files to a Docker image.
-      - GITEA_RUNNER_LABELS=py3.12-node22:docker://git.fabledsword.com/bvandeusen/runner-base:py3.12-node22
       - CONFIG_FILE=/config/config.yml
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock