mac80211: remove legacy minstrel rate control

Now that minstrel_ht supports legacy rates, it is no longer needed

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-4-nbd@nbd.name


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