mac80211: remove sent_ps_buffered

This variable is set once, and tested once.
However, the code path that can set it is
mutually exclusive with the code path that
tests it, so the test is always true. Thus
we also don't need to set it either and can
just remove the variable.

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