nl80211: Remove NL80211_CMD_SET_MGMT_EXTRA_IE

The functionality that NL80211_CMD_SET_MGMT_EXTRA_IE provided can now
be achieved with cleaner design by adding IE(s) into
NL80211_CMD_TRIGGER_SCAN, NL80211_CMD_AUTHENTICATE,
NL80211_CMD_ASSOCIATE, NL80211_CMD_DEAUTHENTICATE, and
NL80211_CMD_DISASSOCIATE.

Since this is a very recently added command and there are no known (or
known planned) applications using NL80211_CMD_SET_MGMT_EXTRA_IE and
taken into account how much extra complexity it adds to the IE
processing we have now (and need to add in the future to fix IE order
in couple of frames), it looks like the best option is to just remove
the implementation of this command for now. The enum values themselves
are left to avoid changing the nl80211 command or attribute numbers.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>