Bug fix when loading manifests with relative path

I tried to serve a local dash stream with a relative path manifest:

```
player.attachSource("example/dash.mpd");
```

The `BaseURL` xml attribute is not set in this particular manifest, so the player tries to compute it manually, ends up with `example/example` and issues invalid segment requests.

I removed those three lines in `DashParser.js` as I don't get their purpose. Please enlighten me if they handle a legitimate situation :).