Don't block deleting non-FxA subscribers

[Vinnl]

Legacy feature flags that were assigned a fake feature flag user in the feature_flag_events table, were associated with a subscriber without an FxA UID. Now that we want to delete those subcribers, the foreign key on this table is blocking it.

Thus, this changes the assciated fake feature flag user.

[github-actions[bot]]

Preview URL :rocket: : https://blurts-server-pr-5905-mgjlpikfea-uk.a.run.app

[Vinnl]

Closing - this would probably still be a good change, but I believe we already deleted these subscribers some other way, so it's maybe no longer worth the review effort.