Bugfix/webvtt-parse: cue offset (subtitle stream without X-TIMESTAMP-MAP)

[mtoczko]

This PR will...

Removes unnecessary code, 1.x.x versions do not need this code, they already have the cueOffset calculation fixed

Why is this Pull Request needed?

subtitle stream without X-TIMESTAMP-MAP, has incorrect cue start/end time

Are there any points in the code the reviewer needs to double check?

Resolves issues:

#4500

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[itsjamie]

Something strange has happened with this branch, all the commits between the master seem to have been duplicated. Could you rebase this to a clean commit with your change?

[itsjamie]

I spot-checked the deploy preview using the original stream that reported the issue and could not reproduce the captions being late.

Additionally, I checked the BipBop stream with has timestamp maps, and all seemed to be well. Assuming we can get a clean source branch, I don't see any reason to not merge.

[mtoczko]

@itsjamie

Something strange has happened with this branch, all the commits between the master seem to have been duplicated. Could you rebase this to a clean commit with your change?

done