Remove dot-com check in certificate cluster migration service

Context

In !178257 (merged) a Gitlab.com? check was added to control which version of the agent was installed. Typically this should be a SaaS-only feature, however as this check is only required until %18.0 (when certificate clusters are disabled on GitLab.com) we used the legacy check and added a Rubocop exception.

Proposal

Remove the return if Gitlab.com? check in Clusters::Migration::InstallService, as it is no longer relevant.