Ignore $context['robot_no_index'] where a canonical URL exists

Why tell robots not to index a page when they could be indexing the
canonical page and potentially earning us some more search prominence?

I looked through all current situations where $context['robot_no_index']
is set and found nothing that could backfire. A fair few of those could
simply be removed as they're now rendered redundant - something to look
into in the future.