mac80211: Remove unused 'rates_idx' variable

Commit f027c2ac introduced 'rates_idx' in
ieee80211_tx_status_noskb but did not use it. Compiling with W=1
gives the following warning, fix it.

mac80211/status.c: In function ‘ieee80211_tx_status_noskb’:
mac80211/status.c:636:6: warning: variable ‘rates_idx’ set but not used [-Wunused-but-set-variable]

This is a harmless warning, and is only being fixed to reduce the
noise generated with W=1.

Fixes: f027c2ac ("mac80211: add ieee80211_tx_status_noskb")
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>