Allow Group owners to delete user's SCIM identities via API

At a minimum we should add a delete endpoint to the SCIM API to allow a Group owner to remove a user's SCIM identity. We've run into a couple of customers with users that previously signed in to SCIM, but now shouldn't. But transparent SSO is now enforcing SCIM for them. This can be resolved by removing the identity, but there's not currently a way for GitLab.com Group owners to remove an identity.

Add a DELETE endpoint so owners can not only GET or PATCH (update), but DELETE, too.

In the future we may like to add these options in the UI, too.

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