Remove unused instant return value from CaptureFuture (#8758)

### Description

Appears to be unused.

### Testing Instructions

```
cargo check
cargo check --tests
```

crates/turbo-tasks/src/capture_future.rs

@@ -51,7 +51,7 @@ pub fn add_allocation_info(alloc_info: AllocationInfo) {
 }
 
 impl<T, F: Future<Output = T>> Future for CaptureFuture<T, F> {
- type Output = (T, Duration, Instant, usize);
+ type Output = (T, Duration, usize);
 
  fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
  let this = self.project();
@@ -68,7 +68,7 @@ impl<T, F: Future<Output = T>> Future for CaptureFuture<T, F> {
  let (duration, allocations, deallocations) = *this.cell.lock().unwrap();
  let memory_usage = (*this.allocations + allocations)
  .saturating_sub(*this.deallocations + deallocations);
- Poll::Ready((r, *this.duration + duration, start + elapsed, memory_usage))
+ Poll::Ready((r, *this.duration + duration, memory_usage))
  }
  Poll::Pending => Poll::Pending,
  }

crates/turbo-tasks/src/manager.rs

@@ -445,7 +445,7 @@ impl<B: Backend + 'static> TurboTasks<B> {
  };
 
  async {
- let (result, duration, _instant, memory_usage) =
+ let (result, duration, memory_usage) =
  CaptureFuture::new(AssertUnwindSafe(future).catch_unwind())
  .await;