res_pjsip_session: Don't add declined stream if one does not exist.

Given a scenario where a session refresh was done with a removed
stream we would always add a removed stream to the outgoing SDP
even if one did not already exist.

This change makes it so that a removed stream is only placed into
the SDP if one already exists.

ASTERISK-28047

Change-Id: Ibb97d21cdeb87a8acae0c720861b0ff255708442