Remove the logic for de-duplicating in-band text track cues

To de-duplicate cues by comparing their properties cannot work, since
scripts can modify cues in small or big ways at any time. For example,
the de-duplication will fail if the id has changed or if the timings
have been slightly shifted.

It must be the demuxer's responsibility to do this, e.g. by keeping
track of the offsets for which cues have already been sourced. This need
not consume a lot of memory, since it could record ranges of offsets.

BUG=270340

Review URL: https://codereview.chromium.org/74623002

git-svn-id: svn://svn.chromium.org/blink/trunk@162245 bbb929c8-8fbe-4397-9dbb-9b2b20218538