Code clean up - remove unused headers from demuxer, unused ffmpeg dependencies, and redundent info from videodecode sample (#71)

[ROCm/rocdecode commit: 071b37b3bd]

projects/rocdecode/samples/videoDecodeFork/README.md

@@ -20,7 +20,7 @@ This sample uses fork() to create multiple processes to decode the same input vi
 
 * [FFMPEG](https://ffmpeg.org/about.html)
   ```
-  sudo apt install ffmpeg libavcodec-dev libavformat-dev libswscale-dev
+  sudo apt install ffmpeg
   ```
 
 ## Build