atrusctl: remove unsued message_loop header.

message_loop.h header is deprecated in libchrome, and not used in
atrusctl either.

BUG=chromium:1094927
TEST=CQ

Change-Id: I8776ef101a727eee5b4000f60aee3d6c14706b72
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/atrusctl/+/2595647
Reviewed-by: Joe Tessler <jrt@chromium.org>
Commit-Queue: Qijiang Fan <fqj@google.com>
Tested-by: Qijiang Fan <fqj@google.com>
diff --git a/src/udev_device_manager.h b/src/udev_device_manager.h
index a18d8b6..a2b3d2e 100644
--- a/src/udev_device_manager.h
+++ b/src/udev_device_manager.h
@@ -9,7 +9,6 @@
 
 #include <base/files/file_descriptor_watcher_posix.h>
 #include <base/macros.h>
-#include <base/message_loop/message_loop.h>
 #include <base/observer_list.h>
 
 #include "scoped_udev_handle.h"