codecs: vp9decoder: Make duplicate_picture() vfunc optional

codecs: vp9decoder: Make duplicate_picture() vfunc optional

The default implementation was required when superframe parsing
was handled by vp9decoder. For instance, if a superframe consists
of multiple frames with show_existing_frame header, it was vague
that which GstVp9Picture should consume GstVideoCodecFrame.

After 1.18 release, we introduced vp9parse element and
superframe should be handled by upstream vp9parse elemenet now.
So, we don't need to care about the superframe at vp9decoder class
level anymore. Simply, a frame corresponding to show_existing_frame
can be dropped if subclass doesn't implement duplicate_picture().

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2151>