mac80211: remove useless setting of IEEE80211_TX_INTFL_DONT_ENCRYPT

There's no value in setting a flag that will
never be checked after this point, this seems
to be legacy code -- I think previously the
flag was used to check whether to encrypt the
frame or not. Now, however, the flag need not
be set, and setting it actually interferes if
the frame will be processed again later.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>