ReplicatedPG: remove unused parameter in function write_update_size_and_usage

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>
Merge pull request #2973 from wonzhq/unused-parm

ReplicatedPG: remove unused parameter in function write_update_size_and_usage

Reviewed-by: Loic Dachary <ldachary@redhat.com>