playbin3: Cleanup and refactor combiner sourcecombine

playbin3: Cleanup and refactor combiner sourcecombine

* Remove fields no longer used, or that can be replaced by smaller code
* Rename "channels" to a more meaningful "input pads"
* Directly handle/use combiner pads in the combiners instead of on the playbin3
  main structure

Remove the corresponding combiner sinkpad whenever a uridecodebin3 source pad
goes away
* If used, store the corresponding combiner sink pad in the SourcePad helper
  structure

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2384>