proxy: Do not set filters for track types we do not have
Since https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/commit/878a1960808d7c4d3c99f6dd3116476ae7e7aef5 the filters are added to the bin right from the start, but we have sinks in our filters, which is a weird use case which would be hard to handle at the GStreamer level. Avoid useless filters in our transcoding pipeline and avoid proxy creation timeouting in our tests.