chore(turbo-tasks): Delete vc generics support (#70817)

The previous PRs in this stack remove the only uses/callsites. This
removes support for vc generics!

There's still a small bit of support left in the form of
`VcValueType::Repr`, but that touches more things, so that'll be a
follow-up PR.

## Why?

Rather than extended support for `Vc` generics to `ResolvedVc`, I plan
to remove support for them entirely. That means fixing all the callsites
(there's not many).

Removing this is needed to get us to a point where 100% of structs can
be `ResolvedValue`, because structs using these fields cannot otherwise
be ported over to `ResolvedVc`.

## Okay, but Why?

Explained here:
vercel/turborepo#8843 (comment)

I expect removing support for this will decrease the overall size of the
codebase, as the logic for supporting generics is rather complicated.

turbopack/crates/turbo-tasks-testing/tests/local_cell.rs

@@ -33,26 +33,6 @@ async fn test_store_and_read() -> Result<()> {
     .await
 }
 
-#[tokio::test]
-async fn test_store_and_read_generic() -> Result<()> {
-    run(&REGISTRATION, || async {
-        // `Vc<Vec<Vc<T>>>` is stored as `Vc<Vec<Vc<()>>>` and requires special
-        // transmute handling
-        let cells: Vc<Vec<Vc<u32>>> =
-            Vc::local_cell(vec![Vc::local_cell(1), Vc::local_cell(2), Vc::cell(3)]);
-
-        let mut output = Vec::new();
-        for el in cells.await.unwrap() {
-            output.push(*el.await.unwrap());
-        }
-
-        assert_eq!(output, vec![1, 2, 3]);
-
-        Ok(())
-    })
-    .await
-}
-
 #[turbo_tasks::function(local_cells)]
 async fn returns_resolved_local_vc() -> Vc<u32> {
     let cell = Vc::<u32>::cell(42);