Cleanup: Drop hobject_incorrect_pool

This field in MOSDSubOp, used to support very old version
OSD(MOSDSubOp header version < 7, which was introduced in
mid 2012 by commit 2fe9816305fc1406ab7fb043bbb32affffc2d190)

Now since two years pasted, we don't need it anymore.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
Merge pull request #3114 from xiaoxichen/drop_incorrect_pool

Cleanup: Drop hobject_incorrect_pool

Reviewed-by: Samuel Just <sjust@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>