Remove an unnecessary comment

Remove an unnecessary 'TODO' comment
because the check is already done
when checking the host existence.

Change-Id: I242159f80a7a682aac4632511fbd3ecc9b2d34db
TrivialFix

nova/compute/api.py

@@ -3506,10 +3506,6 @@ class API(base.Base):
             else:
                 # Set the host and the node so that the scheduler will
                 # validate them.
-                # TODO(takashin): It will be added to check whether
-                # the specified host is within the same cell as
-                # the instance or not. If not, raise specific error message
-                # that is clear to the caller.
                 request_spec.requested_destination = objects.Destination(
                     host=node.host, node=node.hypervisor_hostname)