[MAC80211]: remove turbo modes

This patch removes all mention of the atheros turbo modes that
can't possibly work properly anyway since in some places we don't
check for them when we should.

I have no idea what the iwlwifi drivers were doing with these but
it can't possibly have been correct.

Cc: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>