videoaggregator: remove broken rate adjustment

The start_time and end_time in this context have already been adjusted for the input's rate by converting them to running time above. What is needed afterwards is to compare these with the output's start/stop running time, which also takes into account the rate, so we are comparing equal things.

Multiplying these with the output's rate here is only breaking this logic. In most cases the input and output rate is the same, so this multiplication effectively reverses the rate adjustment that happened while converting to running time, which is why we see the video playing with the original rate in tests.

Fixes #481 (moved)