chore: default remove_dom to false

[Rich-Harris]

A consequence of #12258 is that we only need to remove DOM nodes when a branch or root effect is being destroyed. All other times, there's nothing we need to do.

I suspect we already weren't removing the nodes in question, and so this probably won't actually bring any performance benefits, but it reflects intent more accurately this way.

Made a couple of drive-by style fixes while I was at it.

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