Remove composite identity from Admin area

In the Admin area, when you select GitLab Duo, there is a setting to Turn on /off composite identity

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We needed this trigger to create an instance level oauth application and instance level Service account for composite identity. Now that we are not utilising instance level SA anymore, we can remove this setting since it adds additional confusion. We've moved the creation of oauth application (if not present) to request basis with #585647 (closed). We also need to remove the associated docs page that mention this setting.

If you turn it off, every setting goes away, including those for Classic Duo:

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Related banners/dialog boxes:

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Related docs:

https://docs.gitlab.com/administration/gitlab_duo/configure/gitlab_self_managed/#turn-on-composite-identity

Edited by 🤖 GitLab Bot 🤖