Remove upstreamed <track> and WebVTT tests

https://github.com/w3c/web-platform-tests/commit/ba982a2de52f9650bcb7b503c0c926f847b2c48a
Merge pull request #16 from foolip/rm-track

Remove upstreamed <track> and WebVTT tests

There were reftest conflicts due to pull request #14 from frivoal/reftests