Remove usage of gitlab-shell gitlab-keys script

As mentioned in comments here and here, in 11.10, the authorized_keys file will be managed from gitlab-rails instead of gitlab-shell when the Gitlab.config.gitlab_shell.authorized_keys_file config is set.

Since the goal is to remove the gitlab-keys functionality out of gitlab-shell, we should remove the related code (all of related code are in Gitlab::Shell class) from gitlab-rails and the script (and related classes) from gitlab-shell.