Remove defunct option for building without FFmpeg

Fixes https://bugs.openmw.org/issues/1716

- Added REQUIRED to find_package(FFmpeg)
- Removed USE_FFMPEG option from CMakeLists.txt
- Always use FFmpeg for sound input
- Removed SOUND_DEFINE from CMakeLists.txt
- Removed #else branch from videoplayer.cpp with dummy VideoState code
  (FFmpeg is now guaranteed to exist and the code was incomplete)
- Remove #ifdef OPENMW_USE_FFMPEG in ffmpeg_decoder.cpp, it is guaranteed to be used
- Remove #ifdef OPENMW_USE_FFMPEG from soundmanagerimp.cpp, it is guaranteed to be used