[omxplayer] Remove PCMRemap and handle multichannel mixing like ActiveAE

omxplayer uses pre-ActiveAE code for determining multichannel mixing.
Some of the decisions made with lfe and side/back channels are different.

I think it is better to match latest xbmc code exactly, and so use
ffmpeg to determine the mixing matrix in the same way as Pi Sink does.
Merge pull request #4863 from popcornmix/multichannel_map

[omxplayer] Remove PCMRemap and handle multichannel mixing like ActiveAE