tsdemux: Fix AC-4 detection

This regression was introduced by
gstreamer/gst-plugins-bad!1102
and has been present since 1.18

* Stream Type 0x06 is defined in the base mpeg-ts specification as Private PES
Packets. Determining the content should be solely based on descriptors found
within the PMT.
* This was abused in that commit by defining a "bluray-only" stream type for AC4
: `ST_BD_AUDIO_AC4`
* This should be entirely handled in the regular private pes handling further
down in the code

Fixes #1154

Part-of: <gstreamer/gstreamer!2175>
23 jobs for !2175 with ac4-regression in 117 minutes and 53 seconds (queued for 4 seconds)
merge request