vaapi_decode: Remove vestigial unmap code

The buffer map/unmap code was in an early version of this before it
was committed, but the unmap was never removed.  While wrong, this
was harmless (and therefore unnoticed) because the buffers can't be
mapped at this point - all drivers just did nothing with the call.

libavcodec/vaapi_decode.c

@@ -150,14 +150,11 @@ int ff_vaapi_decode_issue(AVCodecContext *avctx,
 {
     VAAPIDecodeContext *ctx = avctx->internal->hwaccel_priv_data;
     VAStatus vas;
-    int err, i;
+    int err;
 
     av_log(avctx, AV_LOG_DEBUG, "Decode to surface %#x.\n",
            pic->output_surface);
 
-    for (i = 0; i < pic->nb_param_buffers; i++)
-        vaUnmapBuffer(ctx->hwctx->display, pic->param_buffers[i]);
-
     vas = vaBeginPicture(ctx->hwctx->display, ctx->va_context,
                          pic->output_surface);
     if (vas != VA_STATUS_SUCCESS) {