simplify the use of toolexecCmd in reverse

The list flags are entirely unused, so they can be omitted.
The only argument that matters is the package argument to load.
While here, update the TODO, as it no longer applies.

reverse.go

@@ -33,16 +33,9 @@ One can reverse a captured panic stack trace as follows:
 	}
 
 	pkg, args := args[0], args[1:]
-	listArgs := []string{
-		"-json",
-		"-deps",
-		"-export",
-	}
-	listArgs = append(listArgs, flags...)
-	listArgs = append(listArgs, pkg)
-	// TODO: We most likely no longer need this "list -toolexec" call, since
-	// we use the original build IDs.
-	_, err := toolexecCmd("list", listArgs)
+	// We don't actually run `go list -toolexec=garble`; we only use toolexecCmd
+	// to ensure that sharedCache.ListedPackages is filled.
+	_, err := toolexecCmd("list", []string{pkg})
 	defer os.RemoveAll(os.Getenv("GARBLE_SHARED"))
 	if err != nil {
 		return err