lavf: Remove experimental flag for Opus in WebM

Opus in WebM is no more experimental as we have everything necessary in
the container writing code as per the spec. So removing the experimental flag.
Note that we removed the experimental suffix from the CodecId field long
ago ( http://goo.gl/O0TYRB ).

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>