mac80211: Remove unused 'sband' and 'local' variables

Commit b1bce14a (mac80211: update opmode when adding new station)
refactored ieee80211_vht_handle_opmode into __ieee80211_vht_handle_opmode
and ieee80211_vht_handle_opmode leaving a set but unused variable
(sband) in the former. Compiling with W=1 gives the following warning,
fix it.

net/mac80211/vht.c: In function ‘__ieee80211_vht_handle_opmode’:
net/mac80211/vht.c:424:35: warning: variable ‘sband’ set but not used [-Wunused-but-set-variable]

Remove 'struct ieee80211_local* local' as well, it was only used to
set sband.

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

Fixes: b1bce14a ("mac80211: update opmode when adding new station")
Cc: Marek Kwaczynski <marek.kwaczynski@tieto.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kirtika Ruchandani <kirtika@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>