Session::onMigrateCompletedOnPrimary should not update the lastWriteDate field

It currently overwrites the latest transaction's lastWriteDate with that from the oplog being migrated and thus it can move the lastWriteDate back. This could cause sessions to be cleaned earlier than the actual last time they were written to.

Instead, it should just keep the value as is since migrations don't really modify the state of the config.transactions from the point of view of a sharded cluster.