mac80211: remove short slot/short preamble incapable flags

There are no drivers setting IEEE80211_HW_2GHZ_SHORT_SLOT_INCAPABLE
or IEEE80211_HW_2GHZ_SHORT_PREAMBLE_INCAPABLE, so any code using the
two flags is dead; it's also exceedingly unlikely that any new driver
could ever need to set these flags.

The wcn36xx code is almost certainly broken, but this preserves the
previous behaviour.

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