Remove call to Clip::changeLength

Remove the call to `Clip::changeLength` from
`TrackContentWidget::changePosition` as it results in bad performance,
especially when the Song Editor is in auto-scroll mode and the position
is changed very often.

The call in question has set the clip to the length that it already has
which in turn resulted in lots of needless calls to `Song::updateLength`.

It also should not be necessary to update a clip length if all that we
do is change the position of the `TrackContentWidget`.

src/gui/tracks/TrackContentWidget.cpp

@@ -270,8 +270,6 @@ void TrackContentWidget::changePosition( const TimePos & newPos )
 	{
 		Clip* clip = clipView->getClip();
 
-		clip->changeLength( clip->length() );
-
 		const int ts = clip->startPosition();
 		const int te = clip->endPosition()-3;
 		if( ( ts >= begin && ts <= end ) ||