librados: pool async create / delete does not delete completion handle

Backport: dumpling
The pool async delete / create function used to delete the internal
completion object. However, caller still holds the allocated completion
object, which it can't drop a reference to (as it'd try to deallocate
the already freed internal object). This fix removes the internal object
deletion, a following commit will fix a related leak (#6444) by having
the application (radosgw) drop the reference even if got an error.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: drop async pool create completion reference

Fixes: #6444
Backport: dumpling
If pool creation fails (e.g., due to -EEXIST) then we leak the
completion object. Earlier we couldn't just drop the reference, as
librados have already removed the internal completion object. This fix
drop the completion reference even if got an error, which is now
possible.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
PendingReleaseNotes: update regarding librados change

Fix related to issue #6444

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #665 from ceph/wip-6444

Wip 6444
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>