Don't store the qmlscope object in a persistent

Don't store the qmlscope object in a persistent

This doesn't make sense, as the persistent is never being
used. So either the scope object is being used from somewhere
else in which case it'll be referenced and marked from there,
or it's not being used in which case, we might as well collect it.

Change-Id: I56cb45282cbc795d4c59314c10491dedb9b7d478
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>