github-ci: remove CentOS 6

Removed CentOS 6 from packages build jobs as already removed at:

  8b08a3ba ("gitlab-ci: remove CentOS 6")

.github/workflows/centos_6.yml

-name: centos_6
-
-on: [push, pull_request]
-
-env:
-  CI_MAKE: make -f .gitlab.mk
-
-jobs:
-  centos_6:
-    # We want to run on external PRs, but not on our own internal PRs
-    # as they'll be run by the push to the branch.
-    if: github.event_name == 'push' ||
-        github.event.pull_request.head.repo.full_name != github.repository
-
-    runs-on: ubuntu-latest
-
-    strategy:
-      fail-fast: false
-
-    steps:
-      - uses: actions/checkout@v2.3.4
-        with:
-          fetch-depth: 0
-          submodules: recursive
-      - name: packaging
-        env:
-          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
-          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
-          AWS_S3_ENDPOINT_URL: ${{ secrets.AWS_S3_ENDPOINT_URL }}
-          LIVE_REPO_S3_DIR: ${{ secrets.LIVE_REPO_S3_DIR }}
-          RELEASE_REPO_S3_DIR: ${{ secrets.RELEASE_REPO_S3_DIR }}
-          GPG_SECRET_KEY: ${{ secrets.GPG_SECRET_KEY }}
-          GPG_SIGN_KEY: ${{ secrets.GPG_SIGN_KEY }}
-        run: |
-          if ${{ github.event_name == 'push' &&
-              ( github.ref == 'refs/heads/master' ||
-                github.ref == 'refs/heads/1.10' ||
-                startsWith(github.ref, 'refs/heads/2.') ||
-                startsWith(github.ref, 'refs/tags') ) }} ; then
-            sudo apt-get -y update
-            sudo apt-get install -y procmail createrepo awscli reprepro
-            mkdir -p ~/.gnupg
-            echo 'digest-algo sha256' >> ~/.gnupg/gpg.conf
-            OS=el DIST=6 ${CI_MAKE} deploy
-          else
-            OS=el DIST=6 ${CI_MAKE} package
-          fi
-      - name: artifacts
-        uses: actions/upload-artifact@v2
-        if: failure()
-        with:
-          name: centos-6
-          retention-days: 21
-          path: test/var/artifacts