Eliminate stale include of identity_manager.mojom.h

Helps clarify what the actual users are here.

Change-Id: I8554fb008046df105e619a56a8fdd73d8269d9d3
Reviewed-on: https://chromium-review.googlesource.com/c/1477013Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634613}

chromeos/components/drivefs/drivefs_host.h

@@ -20,7 +20,6 @@
 #include "chromeos/components/drivefs/mojom/drivefs.mojom.h"
 #include "chromeos/disks/disk_mount_manager.h"
 #include "components/account_id/account_id.h"
-#include "services/identity/public/mojom/identity_manager.mojom.h"
 
 namespace drive {
 class DriveNotificationManager;