qtqa: Delete $qt_dir only if needed

Fix warning which says that script can't remove path when in it.
In this case the dir was already cleaned and it shouldn't be
deleted again.

Task-number: QTQAINFRA-778
Change-Id: Id22501d8ab886799cf873dc2b4588cc94cfcf612
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>