Remove dead code surrounding `gitlab_ci` settings

Summary

gitlab.yml.example contains entries in gitlab_ci: which no longer hold any value. It appears that these have been deprecated for some time, with the last related database migration being in early 2017.

It appears that these settings can be removed from config/initializers/1_settings.rb, and from gitlab.yml.example

production:
  gitlab_ci:
    add_pusher:
    all_broken_build:

Research

This was discovered during discussion of exposing these settings in our charts/gitlab project.

When I went looking for the usage of these settings, the only references I could find for them was in the example configuration, the migrations, and the initializers/1_settings.rb.

cc @joshlambert