osd: remove unnecessary ref handling in _delete_oid

osd: remove unnecessary ref handling in _delete_oid

Let's consider the following case when handling a delete op.
1. Delete --> whiteouted
2. Make clone

In this case, current code clears chunk_map and calls dec_all_manifest_refcount()
in _delete_oid() even if the clone still has the references.

To fix this, This commit remove unnecessary ref handling in _delete_oid, and
makes finish_ctx() to handle ref handling, which can aware of whether the
clone is created or not.

Also, remove oi.size == 0 condition in finish_ctx() to handle ref. counting
upon a delete op with whitedouted clone.

Signed-off-by: Myoungwon Oh <myoungwon.oh@samsung.com>