rtl8xxxu: remove set but not used variable 'rate_mask'

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:4484:6:
 warning: variable rate_mask set but not used [-Wunused-but-set-variable]

It is never used since commit a9bb0b51 ("rtl8xxxu: Improve
TX performance of RTL8723BU on rtl8xxxu driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -4484,11 +4484,6 @@ static u16
 rtl8xxxu_wireless_mode(struct ieee80211_hw *hw, struct ieee80211_sta *sta)
 {
 	u16 network_type = WIRELESS_MODE_UNKNOWN;
-	u32 rate_mask;
-
-	rate_mask = (sta->supp_rates[0] & 0xfff) |
-		    (sta->ht_cap.mcs.rx_mask[0] << 12) |
-		    (sta->ht_cap.mcs.rx_mask[0] << 20);
 
 	if (hw->conf.chandef.chan->band == NL80211_BAND_5GHZ) {
 		if (sta->vht_cap.vht_supported)