session2: add remove_stream() method

darcs-hash:20090423101114-b2873-d12f3ca944e6ae1aafb5ba7d1c26849f1e7dbeaf
sip_session2: don't fail is one of the streams is None (after removal)

darcs-hash:20090423101135-b2873-ac5082055aafc7f1b87aa673b222bd6699dfb103