[s13n] Remove unused header from ios_chrome_signin_client.mm

This CL removes unused header, device_id_helper.h, from
ios_chrome_signin_client.mm.

Bug: 927332
Change-Id: I34c9fd2254e1b4170f1030d54517d452cb414f6d
Reviewed-on: https://chromium-review.googlesource.com/c/1448008Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#628497}

ios/chrome/browser/signin/ios_chrome_signin_client.mm

@@ -7,7 +7,6 @@
 #include "base/strings/utf_string_conversions.h"
 #include "components/metrics/metrics_service.h"
 #include "components/signin/core/browser/cookie_settings_util.h"
-#include "components/signin/core/browser/device_id_helper.h"
 #include "components/signin/ios/browser/account_consistency_service.h"
 #include "ios/chrome/browser/application_context.h"
 #include "ios/chrome/browser/browser_state/browser_state_info_cache.h"