avformat: implement retiming directly in mxfenc and gxfenc

avformat: implement retiming directly in mxfenc and gxfenc

Generic retime functionality is replaced by a few lines of code directly in the
muxers which used it, which seems a lot easier to understand and this way the
retiming is not dependant of the input durations.

Also remove retimeinterleave, since it is not used by anything anymore.

Signed-off-by: Marton Balint <cus@passwd.hu>