gitaly/config: Stop validating existence of GitLab secret
In 5d7ecbde (gitaly/config: Allow configuration without gitaly-hooks
directory, 2023-08-24), we have changed the validation logic such that
it becomes possible to run without a [gitlab-shell]
section in the
Gitaly configuration. As part of this change we have also started to
verify that the GitLab secret file actually exists on disk. As it turns
out though this breaks misconfigured setups out there where the secret
does not exist.
Let's roll back this validation. While it does detect misconfigured setups, the world doesn't seem to be ready for this change.