The _timeout_async_id field identifies the timer which checks whether
the Gst.MessageType.ASYNC_DONE has been received, implying the pipeline
expects an async-done event.
The _waiting_for_async_done field indicates whether the timer has been
started.

When a timeout occured, the _timeout_async_id is being unset and the
recovery procedure is started. Hopefully this recovery procedure ends up
in an ASYNC_DONE being pushed which leads to _waiting_for_async_done to
be unset.

In practice we don't need both fields. When a timeout occurs, the
recovery procedure should set the state to PAUSED, which means a new
timer is started, so then _timeout_async_id is non-zero.

This commit removes the _waiting_for_async_done field.

Reviewed-by: Thibault Saunier <tsaunier@gnome.org>
Differential Revision: https://phabricator.freedesktop.org/D1550