carl9170: remove lost-frame workaround

This patch removes some cruft, which survived
the RFC review phase.

Originally, carl9170_tx_ampdu_queue erroneously
dropped a lot of frames. As a result the ampdu
scheduler bogged down quite frequently and the
affected BA session timed out.

However this bug has been fixed and the WA and
its debugfs counter is no longer useful.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>