splitmux: Improve handling of repeated timestamps

When handling input with timestamps that repeat, sometimes
splitmuxsink would get confused and ignore a keyframe.

The logic in question is a holdover from before the cmd queue
moved the file cutting to the multiqueue output side and made
it deterministic, so it's no longer needed on the input
here.

https://bugzilla.gnome.org/show_bug.cgi?id=796773
1 file changed