[Segment Replication] Remove unnecessary calls to directory.fsync and directory.syncmetadata

[mch2]

These calls are not necessary with segment replication enabled and at last 1 replica shard. They can be removed. This change should be gated by our index setting.

[mch2]

This issue only applies when using a remote store as the source of replication. When copying from primary shards we still require fsyncs when segments related to a new commit point are received.

[mch2]

Closing, this is disabled with #4402