avformat/matroskaenc: Don't needlessly copy AVCodecParameters

avformat/matroskaenc: Don't needlessly copy AVCodecParameters

At the end of encoding, the FLAC encoder sends a packet whose side data
contains updated extradata (e.g. a correct md5 checksum). The Matroska
muxer uses this to update the CodecPrivate.

In doing so, the stream's codecpar was copied. But given that writing
a FLAC CodecPrivate does not modify the used AVCodecParameters at all,
there is no need to do so and this commit changes this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>