Since the MigrationManager is using clusterId as a lock session id, which is what DistLockHandle is, there is no longer a need for MigrationManager::CollectionMigrationsState to store this value – it will be a constant lockSessionID for every balancer instance: there's a _lockSessionID local variable in MigrationManager.

Remove the MigrationManager::CollectionMigrationsState struct's distLockHandle variable, and clean up the MigrationManager code a bit because it no longer needs to handle it.