Disable `setup_reader` to prevent flaky spec in `lib/gitlab/import_export/project/tree_restorer_spec`

What does this MR do?

Disables setup_reader which potentially cause flaky tests when feature flag misbehaves (Original discussion in #213793 (comment 319730517)).

Without setup_reader, the reader would fall back to legacy_relation_reader, and the test would continue as if it is testing legacy_relation_reader.

#213793 (comment 320801541)

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Does this MR meet the acceptance criteria?

Conformity

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Security

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