rtpjitterbuffer: Don't use estimated_dts to do default skew adjustment

When the buffer DTS is estimated based on arrival time at the jitterbuffer (rather than provided on the incoming buffer itself), it shouldn't be used for skew adjustment. The typical case is packets being deinterleaved from a tunnelled TCP/HTTP RTSP stream, and the arrival times at the jitter buffer are not well enough correlated to usefully do skew adjustments.

This restores the original intended behaviour for the 'estimated dts' path, that was broken years ago during other jitterbuffer refactoring.