Discontinued enca and rtmpdump updates.

Since they are no longer used directly or as dependencies for any other projects it has been decided to no longer provide future updates to both enca and rtmpdump. Enca was only used by libass which dropped this requirement some time ago. Meanwhile, rtmpdump usage stopped over a year ago as ffmpegs native rtmp support has been the preferred option for some time. The previous releases are still available on the Projects page it is just that no future releases will be supplied.