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>