mutation_reader: drop consume_flattened_in_thread()
mutation_reader: drop consume_flattened_in_thread()

duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla