webrtcbin: Improve SDP intersection for Opus

Remove optional sprop-stereo and sprop-maxcapture fields from Opus remote offer caps before intersecting with local codec preferences.

According to https://datatracker.ietf.org/doc/html/rfc7587#section-7.1 those fields are sender-only informative, and don't affect interoperability.

Fixes cases where the webrtc media will end up receive-only if the local side wants to send stereo but the remote is sending mono, or vice versa.

There may be other fields in other codecs, so the implementation anticipates needing to add further fields and codecs in the future.