Remove excess call to invalidate()

QQuickLinearLayout::insertLayoutItem is only called from
updateLayoutItems(), and updateLayoutItems() calls invalidate() after
it has finished calling insertLayoutItem() with all its children.

Change-Id: I67a195522fd7807d77147bc5cfe33e01af105f3f
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>