rtp: Remove the "correction" of timestamps

We tend to comment out this code every XXC3, let's just get rid
off it. The nanoBTS has a rather odd timing behavior. Jacob has
spent hours this summer to trial/document/fix it in the MGCP code
and this code is not getting close to working around the ip.access
audio issues.

If someobody still wants to use the nanoBTS then he should help
to have the MGCP MGW use/share code with the rtp_proxy.c