codecs: Stop claiming constness for refcounted object

codecs: Stop claiming constness for refcounted object

It's almost pointless and makes little sense as subclass might
want to modify refcount of the object or so. And all subclasses
are already casting them to non-const version as well.
In a general sense, we need to avoid passing refcounted object
with const qualifier.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1238>