gitlab_shell['migration'] is removed from 18.0

Epic: gitlab-org&14845 (closed)

Related to gitlab-com/gl-infra/delivery#20558 (closed).

This value was removed from all of our configuration in chef-repo. However, it was still being set to enabled=false here. This is preventing the installation of 18.0 pre-release Omnibus packages on deploy VMs. It might cause more problems in the future because we also use Omnibus for Gitaly in all environments.

This default value was set in !130 (merged). I think we can unset this now that this configuration has been removed completely.