mac80211: minstrel: remove cur_prob from debugfs

This field is redundant, because it is simply last success divided by
last attempt count. Removing it from the rate stats struct saves about
1.2 KiB per HT station.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>