webrtcsink: don't return value from error closure

the signal doesn't expect a return value, which meant we were panicking
as soon as the signaller tried to report an error.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1167>

net/webrtc/src/webrtcsink/imp.rs

@@ -1415,7 +1415,6 @@ impl WebRTCSink {
                         gst::StreamError::Failed,
                         ["Signalling error: {}", error]
                     );
-                    false
                 })
             ),