splitmuxsrc: Convert part reader to a bin with a non-async bus

splitmuxsrc: Convert part reader to a bin with a non-async bus

A pipeline always has an async bus, which involves allocating an fd pair. As
splitmuxsrc only uses the bus' sync handler, this is not required and can easily
cause splitmuxsrc to exceed the fd limit for no good reason.

The other features of GstPipeline are also not needed here, e.g. clock selection.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7957>