ffv1dec: Drop rank for now

We'll keep the rank lower than avdec_ffv1, at least until we're
comfortable with support for the entire range of possible inputs working
well.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1174>

docs/plugins/gst_plugins_cache.json

@@ -1702,7 +1702,7 @@
                         "presence": "always"
                     }
                 },
-                "rank": "primary + 1"
+                "rank": "none"
             }
         },
         "filename": "gstffv1",

video/ffv1/src/ffv1dec/mod.rs

@@ -21,7 +21,7 @@ pub fn register(plugin: &gst::Plugin) -> Result<(), glib::BoolError> {
     gst::Element::register(
         Some(plugin),
         "ffv1dec",
-        gst::Rank::Primary + 1,
+        gst::Rank::None,
         Ffv1Dec::static_type(),
     )
 }