mac80211: Remove unused 'beaconint_us' variable

Commit 4a733ef1 (mac80211: remove PM-QoS listener) removed all use
of 'beaconint_us' from ieee80211_recalc_ps() but left the variable
intact. Compiling with W=1 gives the following warning, fix it.
net/mac80211/mlme.c: In function ‘ieee80211_recalc_ps’:
net/mac80211/mlme.c:1481:7: warning: variable ‘beaconint_us’ set but not used [-Wunused-but-set-variable]

iee80211_tu_to_usec has no side-effects and is safe to remove.

Fixes: 4a733ef1 ("mac80211: remove PM-QoS listener")
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kirtika Ruchandani <kirtika@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -1486,10 +1486,6 @@ void ieee80211_recalc_ps(struct ieee80211_local *local)
 
 	if (count == 1 && ieee80211_powersave_allowed(found)) {
 		u8 dtimper = found->u.mgd.dtim_period;
-		s32 beaconint_us;
-
-		beaconint_us = ieee80211_tu_to_usec(
-					found->vif.bss_conf.beacon_int);
 
 		timeout = local->dynamic_ps_forced_timeout;
 		if (timeout < 0)