Remove "show_closed_captions" in cookie

[myleshk]

Storing caption status in global cookie is wired. Below are my reasons:

1. This cookie is only useful when you watch the same video once and refresh/close the page and then come back to watch again, which does not make much sense.
2. Current implementation of this cookie is global, and will override/interfere with other videos in the same domain (not only same course).
3. Because of 2, if the other video does not have transcripts(captions), the toggle button of captions will not show up, so there is no way of closing the captions even during video playing. See below.

Please check this problem. Thx.

[openedx-webhooks]

Thanks for the pull request, @myleshk! I've created OSPR-1519 to keep track of it in JIRA. JIRA is a place for product owners to prioritize feature reviews by the engineering development teams.

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[gsong]

jenkins run all

[myleshk]

Just made some new changes that should fix relevant test failures.

Not sure for other test failures about "problems" as I just changed one JavaScript file of video player.

Please run tests again, thank you.

[shaunagm]

jenkins run all

[cptvitamin]

@myleshk I believe #2 is the desired behavior. If you are hearing impaired for one course, you are likely hearing impaired for all courses.

WRT #3, all videos should have captions defined in Studio.

[myleshk]

I'm closing this because the major problem this PR tries to solve is cause by my ignorance of edX MDC.

However, for Problem 2 I still think it is bothering ordinary users with occasional needs to use captions, e.g. due to noisy environment, unfamiliar terms in one video.