Fix flaky test for StreamingSerializer

Related to #390356 (closed)

What does this MR do and why?

This MR fixes the flaky spec for inaccessible associations export that was failing because it deleted an item from the collection using its index (that might vary) instead of its ID.

$ bin/rspec './spec/lib/gitlab/import_export/json/streaming_serializer_spec.rb[1:1:6:2:1:1:1:1]' --seed 56182

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