OpTracker: clarify that unregister_inflight_op is only called if enabled

The !tracking_enabled branch actually had a leak which was unreachable
since the caller does the check for tracking_enabled.

Signed-off-by: Samuel Just <sam.just@inktank.com>
TrackedOp: clear_payload as well in unregister_inflight_op

We want to minimize the cost of maintaining the historic ops.

Signed-off-by: Samuel Just <sam.just@inktank.com>
working