mac80211: remove erroneous comment about RX radiotap header

There's no way the driver can pre-build the radiotap header,
so remove the comment stating that it can.
Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -66,10 +66,6 @@
  *
  * Secondly, when the hardware handles fragmentation, the frame handed to
  * the driver from mac80211 is the MSDU, not the MPDU.
- *
- * Finally, for received frames, the driver is able to indicate that it has
- * filled a radiotap header and put that in front of the frame; if it does
- * not do so then mac80211 may add this under certain circumstances.
  */
 
 /**