remove gitlab-shell config

Maina Ng'ang'arequested to merge
remove-gitlab-shell-config into master

GitLab shell config were migrated from ruby to Go and the configs are no longer used. See comment. We are setting the default value for gitlab shell migration to false in order to remove the configurations from our chef roles.

Related Issue: gitlab-com/gl-infra/delivery#20558 (closed)

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