Remove div.content from block.html.twig and node.html.twig

There is a todo in block.html.twig:

remove the div around content - and make sure this doesn't break any CSS defined in modules.

Note that there is no similar todo in the node.html.twig, so, to me, this is introducing inconsistencies into how we're structuring various different containers, which at the end of the day, are very similar structures: a wrapper tag, an h2/h3 for the node/block title and a div.content wrapper.

But obviously, I'm open to being convinced otherwise, hence I've opened this issue to discuss and am linking to it in the front-end branch.

This is a follow-up from #1843092: id attribute block.html.twig and node.html.twig.