Use default cloning depth for the Danger CI job

Use default cloning depth for the Danger CI job

With shallow fetching working reliably in pygit2 1.17.0+, there is no
longer any need for GitLab CI runners to clone the BIND 9 repository
with a fixed depth of 1000 during every "danger" CI job as Hazard is now
able to fetch remote refs with an arbitrary depth, controlled by the
HAZARD_FETCH_DEPTH environment variable.  The latter can be defined via
GitLab project's CI settings and adjusted as needed over time, without
the need to update .gitlab-ci.yml every time its value needs to be
changed.

(cherry picked from commit e39e7afc16fe1575afb907be76e55942925e1a3d)