Remove first_rtcp_ntp_time. This is used to prevent overflow of the timestamp,
but doesn't actually do that. What's worse, it creates timestamp adjustments
that are different per stream within a session, leading to a/v sync issues.

See discussion in thread "[FFmpeg-devel] rtp streaming x264+audio issues (and
some ideas to fix them)". Patch suggested by Luca Abeni <lucabe72 email it>.

Originally committed as revision 21857 to svn://svn.ffmpeg.org/ffmpeg/trunk
