Remove bundled MySQL client and related docs
Closes: #4430 (closed)
Depends on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29608 (Because we are dropping installation of mysql group of gems during gitlab-rails installation. I assume that to cause problems if https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29608 isn't merged)
Edited by GitLab Release Tools Bot