chore: simplify effect.dom stuff

[Rich-Harris]

Poking at #11690 some more: I think one of the core ideas there, of assigning effect.dom inside append when not hydrating (rather than doing it in push_template_node) makes a lot of sense. It turns out we're doing stuff that isn't really necessary, and we can make our lives simpler. This also makes the jump from main to #11690 slightly smaller

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[trueadm]

This seems to cause a memory leak. We shouldn't reference all child nodes, as those nodes might get removed from internal effects – yet still get referenced here and thus retained. I should have noticed this on my review, sigh.