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.