chore: more hydration stuff

[Rich-Harris]

another small PR that tidies things up a bit:

• after recent hydration changes, hydrate_block_anchor is only ever called with a Comment during hydration, which means we can simplify that function
• the cleanup (node.$$fragment = undefined) should go in the place where it is initially set, not in an unrelated function

As a result of this change, hydrate_block_anchor and update_hydrate_nodes are almost identical, so I'm not 100% clear on why we have both, but I figured we could make this innocuous change as a starting point

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