audiodecoders: use default pad accept-caps handling
audiodecoders: use default pad accept-caps handling

Avoids useless check of downstream caps when handling an
accept-caps query

Elements: dtsdec, faad, gsmdec, mpg123audiodec, opusdec,
          sbcdec, adpcmdec, sirendec

StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good