Remove VDPAU

I think we should just remove the VDPAU plugin.

It's been replaced by NVENC/NVDEC and even NVIDIA doesn't support VDPAU any longer and hasn't for quite some time.

It's fair to say it's pretty much unmaintained and unsupported and given the track record over the last 10 years it seems highly unlikely anyone is going to make it work well or us adding plumbing for proper zero-copy or gst-gl integration.

I think it's better to remove badly supported things and give clear signals what people should be using instead.