mutation_reader: evictable_reader: remove next_partition() workaround
mutation_reader: evictable_reader: remove next_partition() workaround

`next_partition()` now returns a future<>, so we can forward it to the
remote shard in the scope of the next partition call, remove the
now obsolete workaround for the synchronous next partition.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla