Fix clippy lint

Summary:
clippy complains with

```
non-binding `let` on a future
consider awaiting the future or dropping explicitly with `std::mem::drop`
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future
`#[warn(clippy::let_underscore_future)]` on by default
```

Because the future we are not awaiting comes from `tokio::spawn`, it is still executing. Use `std::mem::drop` to make the intent of ignoring the handle explicit while satisfying clippy.

Reviewed By: yancouto

Differential Revision: D43468794

fbshipit-source-id: 9da8fe940e00c3043fdfba7d25c8add94ca6818e

eden/mononoke/bookmarks/warm_bookmarks_cache/lib.rs

@@ -819,7 +819,9 @@ impl BookmarksCoordinator {
                     self.live_updaters,
                     self.warmers,
                 );
-                let _ = tokio::spawn(async move {
+                // By dropping the future output by tokio::spawn instead of awaiting it, we
+                // delegate the responsibility of managing it to tokio's async queue
+                std::mem::drop(tokio::spawn(async move {
                     let res = single_bookmark_updater(
                         &ctx,
                         &repo,
@@ -849,7 +851,7 @@ impl BookmarksCoordinator {
                             live_updaters.insert(book.key().clone(), state.into_finished(&res));
                         }
                     });
-                });
+                }));
             }
         }