In SERVER-128437 we recently added a LegacyShardCacheCollectionsPresent check to checkMetadataConsistency to assert config.cache.* collections do not exist when upgraded to FCV 9.0. This fully supersedes the assert_no_legacy_shard_cache_collection.js test hook we added in SERVER-127979, so we can just remove it.

This will also fix spurious failures of this hook in the sharding_stepdown_fcv_upgrade_downgrade_jscore_passthrough suite since it's missing logic to be resilient to stepdowns.