rtwn: remove unused rate configuration code for management traffic.

[adrian]

Summary

After discussion with the rtlwifi maintainers, it looks like this
path isn't even used.

(And it's part of the firmware rate control path which we currently
don't enable for other reasons.)

[emaste]

LGTM, I don't think disabled code full of XXX comments has much value and it's not going to help much in whatever we might need to implement here.