Remove useRelativeCueTimestamps option

[joeyparrish]

In v2.0.1, we introduced the useRelativeCueTimestamps option to control the behavior of our WebVTT parser. We decided in #480 (comment) that we would remove this option in v2.1.0. All WebVTT timestamps in v2.1.0 will be relative to the segment time.

[TheModMaker]

We also have a custom asset Tears of Steel (multiperiod with segmented subtitles and PTO) which uses Period-relative timestamps (IIRC). This was added to specifically test the old behavior and should be updated to match the new behavior. (I was the one who created this asset, so I can help)

[joeyparrish]

That asset is still referenced by existing versions, so any updates should be made by forking that asset for v2.1.0 and leaving the existing one where it is.