fix: Only cancel background work if primary RocksDB instance is dropped (#2918)

This effectively reverts
https://github.com/FuelLabs/fuel-core/pull/2730/files which causes a lot
of "Shutdown in progress: stalled writes" errors when multiple rocksdb
instances are being dropped in a running node.

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Co-authored-by: green <xgreenx9999@gmail.com>
Co-authored-by: Aaryamann Challani <43716372+rymnc@users.noreply.github.com>