WIP: Remove ruby code

Part of #173 (closed)

The Ruby code is no longer needed, so remove it.

Some build and install-time dependencies still use Ruby, so it remains as a dependency in .gitlab-ci.yml. We have #213 (closed) to address that.

I need to ensure that there are no test gaps introduced by deleting all these Ruby specs. I need to reimplement specs that are currently in:

• spec/gitlab_shell_authorized_keys_check_spec.rb
• spec/gitlab_shell_authorized_principals_check_spec.rb
• spec/gitlab_shell_custom_git_receive_pack_spec.rb
• spec/gitlab_shell_gitlab_shell_spec.rb
• spec/gitlab_shell_lfs_authentication_spec.rb
• spec/gitlab_shell_two_factor_recovery_spec.rb