Remove obsolete static generation plumbing (#96563)
## Summary
- Removes the unused `RenderOptsPartial.isStaticGeneration` option and
its debug-render write. `WorkStore` derives this value independently.
- Removes `shouldGenerateStaticFlightData` and its redundant guards.
These checks only occur inside `prerenderToStream`, which is exclusively
entered when `isStaticGeneration` is true.
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