bluetooth/gst: Replace buffer accumulation in adapter with direct pull

Bluetooth codecs should always have fixed in/output and are hence able
to have their results directly read from the codec, instead of
accumulating in a buffer asynchronously that is subsequently only read
in the transcode callback.  The Bluetooth backends calling encode/decode
also expect these fixed buffer sizes.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/494>

src/modules/bluetooth/a2dp-codec-gst.h

@@ -46,7 +46,6 @@ struct gst_info {
     /* The appsink element that accumulates encoded/decoded buffers */
     GstElement *app_sink;
     GstElement *bin;
-    GstAdapter *sink_adapter;
     /* The sink pad to push to-be-encoded/decoded buffers into */
     GstPad *pad_sink;