Update GitLab-Shell install script to not touch Gitaly owned directories

Source: https://gitlab.com/gitlab-org/gitlab-shell/blob/48142b98ca9565698632c44e72e29e22b9c923c3/bin/install#L18

When GitLab Shell is installed, it will create the storage locations and set the correct mode for the directory. That is something Gitaly should do as that's Gitaly's responsibility.