mpegdemux cleanups
Get rid of a few debug/warning messages during normal operation:
DEBUG mpegpsdemux gstmpegdemux.c:577:gst_ps_demux_get_stream_from_pad:<mpegpsdemux0:audio_c0> no stream found for pad!
and
WARN default descriptions.c:746:format_info_get_desc: Unexpected MPEG-1 layer in audio/mpeg, mpegversion=(int)1