refactor: move PlayFlags defaults into Playbin3 and clean up unused prelude imports

gst/src/plugins/app/appsink.rs

@@ -16,7 +16,6 @@ impl AppSink {
     }
 
     pub fn new(name: impl AsRef<str>) -> Result<Self> {
-        use gstreamer::prelude::*;
         let inner = gstreamer::ElementFactory::make("appsink")
             .name(name.as_ref())
             .build()
@@ -133,7 +132,7 @@ fn test_appsink() {
             .build(),
     );
 
-    let mut video_sink = video_convert
+    let video_sink = video_convert
         .link(&appsink)
         .expect("Link videoconvert to appsink");