gitaly/config: Stop validating existence of GitLab secret file and custom hooks dir

In 5d7ecbde (gitaly/config: Allow configuration without gitaly-hooks directory, 2023-08-24), we have introduced a that allows us to get rid of the [gitlab-shell] section in favor of explicitly configured options. As part of this change we also introduced validation of whether the now explicitly configured GitLab secret file actually exists on disk. As any setup that doesn't have the file is essentially broken we thought this to be a good idea.

There was quite some fallout caused by this change though, mostly in test setups where we don't care about whether Gitaly is fully functional or not. So let's revert this change and stop verifying the existence of the GitLab secret file.

The same is done for the custom hooks directory.

Closes #5563 (closed).