mac80211: remove unused declaration of struct sta_attribute.

This patch removes unused definition of struct sta_attribute
in net/mac80211/ieee80211_i.h.

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