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>
Git-commit: a6bcda44843c6dfced0fb973e2607c2a98addfa9
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-fixed: 2150289
[pxu@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Peng Xu <pxu@codeaurora.org>
Change-Id: I4c2a411da356febcfbb7d511f49d600da3363ba8