Remove unused option from ObjectReplicator.__init__

disk_chunk_size is unused by the ObjectReplicator class. Note that it
*is* used by the object replicator process; it's just that
DiskFileManager is the one that pulls it out of the conf and pays
attention to it. Keeping it as an attribute on ObjectReplicator is
unnecessary.

Change-Id: I1eeef7b33873b4c8bb269ca02dcb067098b6fded
Merge "Remove unused option from ObjectReplicator.__init__"