demux_mf, stream_mf: change license to LGPL

cehoyos, who did not agree to the relicensing, added bcb5c78ce3. If
there was copyright, we consider it gone, because the table changed. It
does not map file extension to a FourCC anymore, and codecs.conf is
gone. The new mapping is a libavcodec codec name (happens to be the same
as the file extension).

The same applies to commits 60ecafec, b749836b, 5b3e3be1. None of these
authors were contacted. These were before the code was replaced with a
table (in d0326807). The parts outside of demux_mf.c were removed a long
time ago. Like in the previous comment, we don't think any copyright
applies at least to the new code (at least after the FourCC removal).

iive authored 0aa37a0d, which is probably still left in some form, and
makes demux_mf.c "LGPL 3 or later".

stream_avdevice.c (unrelated) has been marked as LGPL before.