avutil/hwcontext_vulkan: Fix memleaks when transfer to vulkan

Without ff_vk_exec_discard_deps which is called by ff_vk_exec_wait,
the reference count of hwframe context cannot reach zero due to
circular reference created by ff_vk_exec_add_dep_frame.

Fix #10873
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavutil/hwcontext_vulkan.c

@@ -3394,9 +3394,7 @@ static int transfer_image_buf(AVHWFramesContext *hwfc, AVFrame *f,
     if (err < 0)
         return err;
 
-    /* Wait for the operation to complete when downloading */
-    if (to_buf)
-        ff_vk_exec_wait(&p->vkctx, exec);
+    ff_vk_exec_wait(&p->vkctx, exec);
 
     return 0;
 }