Fix CSCore.Ffmpeg lgpl license as it's gpl right now

[kamyker]

Main readme says: " CSCore.Ffmpeg project which is licensed under the LGPL".

This file in CSCore.Ffmpeg says: "GNU GENERAL PUBLIC LICENSE"

And in general I found out that "The combination of GPL code and LGPL code must be licensed under the GPL." This means that CSCore.Ffmpeg can't use LGPL license.

If FFmpeg is built for LGPL all GPL licenses should be deleted in https://github.com/filoe/cscore/tree/master/CSCore.Ffmpeg/FFmpeg/licenses as those features/libraries are not used.