Remove security_policies_group_transfer_sync feature flag

Dominic Bauerrequested to merge
557122-ff-security_policies_group_transfer_sync-synchronizes-security-policies-after-a-group-was into master

What does this MR do and why?

Removes security_policies_group_transfer_sync feature flag introduced by !199108 (merged).

The feature flag is globally enabled on GitLab.com: https://gitlab.slack.com/archives/C101F3796/p1755593809283159

References

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Related to #557122 (closed)

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