cfg80211: remove unused function ieee80211_data_from_8023()

This function hasn't been used since the removal of iwmc3200wifi
in 2012. It also appears to have a bug when qos=True, since then
it'll copy uninitialized stack memory to the SKB.

Just remove the function entirely.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>