sidxindex: Don't adjust the Period start time depending on the track start time

This was only necessary to get playback to start with dash.js 1.2.0,
it has been fixed in the git version.

The previous behaviour was incorrect - the Period's start time
is irrespective of the actual first timestamp of the contents
within the period. The Period start time only says when, within the
global timeline, this particular piece should start to be played
back.

Signed-off-by: Martin Storsjö <martin@martin.st>