Remove gitlab_shell custom_hooks_dir setting

What does this MR do?

Remove gitlab_shell custom_hooks_dir setting

- Remove the custom_hooks_dir from the gitlab_shell configuration tree

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6393

Changelog: removed

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Related issues

Closes #6393 (closed)

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened