setup: Remove outdated comment about deref &PathBuf to &Path

The double deref `&**src` was removed in CL:1501652.

TEST=cargo check

Change-Id: I9104e159c1af2ab730f47b053e15492378dbe355
Reviewed-on: https://chromium-review.googlesource.com/1504776
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: David Tolnay <dtolnay@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

src/linux.rs

@@ -611,7 +611,7 @@ fn create_9p_device(cfg: &Config, chronos: Ids, src: &Path, tag: &str) -> Device
         }
         None => {
             // There's no bind mount so we tell the server to treat the source directory as the
-            // root.  The double deref here converts |src| from a &PathBuf into a &Path.
+            // root.
             (None, src)
         }
     };