Remove cue position alignment by adding 50 for Firefox

[mackode]

This PR will...

Remove IMO unnecessary cue position alignment.

Why is this Pull Request needed?

Cues are wrongly positioned in current Firefox. Why we use const (50) for Firefox?

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

Yes, if we keep backward compatibility, maybe this is still needed on older Firefoxes.

Resolves issues:

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]

The default value for cue positions must have changed at some Firefox version.

[michaelcunningham19]

Briefly tested this out with some assets with subtitles. It works fine in Firefox without this browser-specific compensation.

However, the clamping rule (0-100) is still enforced by the browser(s). From my testing on Firefox 66 and Chrome 73, both throw exceptions when setting position out of that range.

It would be nice to know more about this Firefox-specific compensation.. @johnBartos do you have any details to share there? You were the last one to make a change in this code: b7f8468 - by adding the clamp 0 <-> 100

[johnBartos]

@michaelcunningham19 It looks like this code should have never been committed as such: jwplayer#87. We removed this from our fork long ago, but neglected to upstream that PR. Thanks for fixing this @mackode