wifi: mac80211: minstrel_ht: remove unused has_mrr member from struct minstrel_priv

Remove unused has_mrr (has multi-rate retry capabilities) member
from struct minstrel_priv (only set once in minstrel_ht_alloc, never
used again).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>