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@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>