Delete some dead turbo task functions (#92596)
Removes a handful of dead code discovered while exploring deletion of the `ValueDefault` trait. That larger change turned out not to be worth it from a binary size perspective, so it was backed out — but the dead code found along the way is worth keeping.
**Removed:**
- `impl ValueDefault for VersionedContentMap` (not tagged with `#[turbo_tasks::value_impl]`, never a turbo-tasks singleton)
- `impl ValueDefault for Body` in `turbopack-dev-server` (same — plain Rust impl, no callers via `Vc::default()`)
- `EmotionTransformConfig::default_private()` and `impl ValueDefault for EmotionTransformConfig` (unused)
- `StyledComponentsTransformConfig::default_private()` and `impl ValueDefault for StyledComponentsTransformConfig` (unused)
- `EmittedEvaluatePoolAssetsWithEffects` struct in `evaluate.rs` (defined but never referenced)
- `OutputAsset::size_bytes()` default trait method (no call sites)
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