vnc: simple clean up

It is unnecessary to assign 'packed_bytes' to 'estimated_bytes', because 'estimated_bytes' unused after assignment.

Signed-off-by: Wei Qi <weiqi4@huawei.com>
Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>