mac80211: minstrel_ht: clean up CCK code

- move ack overhead out of rate duration table
- remove cck_supported, cck_supported_short

Preparation for adding OFDM legacy rates support

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-2-nbd@nbd.name


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