cfg80211: Remove unused wiphy flag

The only user of WIPHY_FLAG_SUPPORTS_SEPARATE_DEFAULT_KEYS was removed
and consequently, this flag can be removed, too. In addition, a single
capability flag was not enough to indicate this capability clearly since
the device behavior may be different based on which operating mode is
being used.

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