revert #8131

[Rich-Harris]

This reverts the optimisation wherein pages with ssr = false and no explicit prerender = false would automatically be prerendered, as it turns out to be unsafe.

• closes '$env/dynamic/public' variables don't get loaded if ssr = false. #9275
• closes Headers are not set when ssr = false #8428

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[benmccann]

This needs a changeset

[dummdidumm]

Honestly I'm a bit relieved this goes away, while that code was constrained to specific parts it was hard to reason about.