Remove restrict_invites_for_comp_id_service_accounts FF

What does this MR do and why?

Related to #581714 This change removes a feature flag that was controlling restrictions on service account invitations (restrict_invites_for_comp_id_service_accounts).

It only affects gitlab.com.

The FF was enabled globally on gprd for more than 1w already.

The code now permanently enables these restrictions - meaning composite identity service accounts can only be invited to groups within their origin hierarchy.

The feature flag configuration file was deleted, all the conditional checks for the flag were removed from the code, and the related test cases that verified the "disabled" behavior were also removed.

How to set up and validate locally

N/A - it is already enabled

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