LinuxKPI: 802.11: remove ieee80211_node_get_txrate_dot11rate()

net80211 ieee80211_node_get_txrate_dot11rate() does not yet support
VHT and that means we get a lot of log lines when doing VHT for a
debugging statement even if it's not active.
Remove the calls to ieee80211_node_get_txrate_dot11rate() as with
hw/fw based rate control (modern iwlwifi) they are pointless anyway
as they are currently.

Sponsored by: The FreeBSD Foundation
X-MFC: only if net80211 gets MFCed
Reported by: J.R. Oldroyd (jr opal.com)
Fixes: 706745001093 (I should have noticed during review)