Remove unncessary group SAML identity assignment

What does this MR do?

Follow-up to !61717 (merged).

As previously implemented the code was technically buggy and could cause confusion in the future. That is because the call to self.identity, if an update were necessary, would fail to update user identities association and would not be auto-saved.

Actually, it wouldn't be possible for the identity to be found in this manner if the extern_uid had changed. Make it clearer by using a guard to indicate nothing is happening in this case.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Security

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