avfilter/dnn_backend_tf: Fix free context at random place

It will be freed again by ff_dnn_uninit.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
Reviewed-by: Wenbin Chen <wenbin.chen@intel.com>
Reviewed-by: Guo Yejun <yejun.guo@intel.com>

libavfilter/dnn/dnn_backend_tf.c

@@ -804,7 +804,7 @@ err:
     if (ff_safe_queue_push_back(tf_model->request_queue, request) < 0) {
         destroy_request_item(&request);
     }
-    dnn_free_model_tf(&tf_model->model);
+
     return ret;
 }