ioat: do not perform removal actions at shutdown

Unregistering services should only happen at "remove" time.  This prevents
the device from being unregistered while dmaengine clients are still
active.  Also, the comment on ioat_remove is stale since removal is prevented
while a channel may be in use.

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>