QAlsaAudioInput remove unused async_handler member.

The member 'ahandler' was not used and will not be used. Removing to
avoid confusion.

Change-Id: I35f3659a650f109c564c6308573a211f324e5411
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
QAlsaAudioOutput remove async callback.

The async callback mechanism in ALSA is prone to deadlock.
There was already a timer fallback mechanism that appears to be
sufficient.

Task-number: QTBUG-39677
Change-Id: I44b59e6b16eea1c9c4eeb6967335ce4f468cf3c4
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>