[ci] remove job github-sync (use gitlab mirroring instead)

.gitlab-ci.yml

 stages:
-  - github-sync
   - build
   - test
   - integration
@@ -7,27 +6,6 @@ stages:
   - prerelease
   - release
 
-push_to_github:
-  stage: github-sync
-  variables:
-    GIT_STRATEGY: none
-  tags:
-    - github
-  script:
-    - rm -rf ./*
-    - rm -rf .git
-    - git clone --mirror $CI_REPOSITORY_URL
-    - git remote add github $GITHUB_URL_AND_KEY
-    - git config --global user.email "contact@duniter.org"
-    - git config --global user.name "Duniter"
-    # Job would fail if we don't remove refs about pull requests
-    - bash -c "cat packed-refs | grep -v 'refs/pull' > packed-refs-new; echo 'Removed pull refs.'"
-    - mv packed-refs-new packed-refs
-    - bash -c "git push --force --mirror github 2>&1 | grep -v duniter-gitlab; echo $?"
-  only:
-    - master
-    - dev
-
 .changes: &changes
   only:
     changes: