Don't show Duo Agent platform settings on self managed

The following discussion from !197439 (merged) should be addressed:

• @bastirehm started a discussion: (+1 comment)

  @fdegier are these settings now shown by default? If so, can we hide them again for 18.3?

  Composite Identity usage unfortunately had to be postponed (see https://gitlab.com/gitlab-org/gitlab/-/issues/557363) and the setting referenced here is only about creating the service account. For now for end-users a project setting is more important: https://gitlab.com/gitlab-org/gitlab/-/issues/559240.

  Can you create a separate issue and add it to the current milestone/iteration if we need to hide something again for the moment?