res/res_pjsip_sdp_rtp: Undo 425921

This patch for r425921 introduced a different bug, wherein sending an INVITE
request with no SDP would cause Asterisk to not send an SDP Offer in the 200
OK. The current structure of res_pjsip_sdp_rtp is a bit hard to deal with
to fix this, particularly in 12:
(1) The format capabilities structures and how they are used are a bit harder
    to manipulate than they are in 13
(2) create_outgoing_sdp has no knowledge of whether or not it is creating an
    SDP as a new Offer or an Answer. This is something of an oversight in the
    callback definition, as the caller of it does have this information.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@425943 f38db490-d61c-443f-a65b-d21fe96a405b