lavf/matroska: Removing experimental suffix for Opus

Removing the /EXPERIMENTAL suffix for codec id field for Opus. Doing this
as the implementation of embedding Opus in Matroska is complete.

Signed-Off By: Vignesh Venkatasubramanian <vigneshv@google.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>