mac80211: minstrel_ht: remove unused variable 'mg'

This probably came in through some refactoring and what is
now a call to minstrel_ht_group_min_rate_offset(), remove
the unused variable.
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210219105744.f2538a80f6cf.I3d53554c158d5b896ac07ea546bceac67372ec28@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -805,7 +805,6 @@ minstrel_ht_group_min_rate_offset(struct minstrel_ht_sta *mi, int group,
 static u16
 minstrel_ht_next_inc_rate(struct minstrel_ht_sta *mi, u32 fast_rate_dur)
 {
-	struct minstrel_mcs_group_data *mg;
 	u8 type = MINSTREL_SAMPLE_TYPE_INC;
 	int i, index = 0;
 	u8 group;
@@ -813,7 +812,6 @@ minstrel_ht_next_inc_rate(struct minstrel_ht_sta *mi, u32 fast_rate_dur)
 	group = mi->sample[type].sample_group;
 	for (i = 0; i < ARRAY_SIZE(minstrel_mcs_groups); i++) {
 		group = (group + 1) % ARRAY_SIZE(minstrel_mcs_groups);
-		mg = &mi->groups[group];
 
 		index = minstrel_ht_group_min_rate_offset(mi, group,
 							  fast_rate_dur);