footageviewer: remove unnecessary cache invalidations

Minor optimization, now that the FootageViewer's nodes are
connected/disconnected as part of the process, the second call to invalidate
is unnecessary and will only serve to re-invalidate actually valid cache data.

app/widget/viewer/footageviewer.cpp

@@ -64,8 +64,6 @@ void FootageViewerWidget::SetFootage(Footage *footage)
     }
 
     ConnectViewerNode(viewer_node_, footage->project()->color_manager());
-    video_renderer_->InvalidateCache(TimeRange(0, viewer_node_->Length()));
-    audio_renderer_->InvalidateCache(TimeRange(0, viewer_node_->Length()));
   }
 }