timelinepipeline: fix caps leak
timelinepipeline: fix caps leak

cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,pitivi/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,pitivi/gst-editing-services