gnu: Remove ffmpeg@3.3.

The reason this was required was because of a regression in ffmpeg 3.4; see
<https://trac.ffmpeg.org/ticket/6775>.

* gnu/packages/gstreamer.scm (gst-libav)[inputs]: Change FFMPEG-3.3 to FFMPEG.
* gnu/packages/video.scm (ffmpeg-3.3): Remove variable.