Clean up subtitle cleanup

[Tom Macdonald]

The cues are not being cleaned up properly, so we attach to several unrelated player events and call the subtitbleClearHandler callback on each one in the hopes that subtitles will be cleared properly.

We should come up with a better way to handle this cleanup, and remove the event listeners.

This was already addressed once in a3087c6#diff-721435a6b33220240fadb91c3a0f3323 but that commit was dropped as it doesn't solve the problem.