[s13n] Remove unused header from ios_web_view_signin_client.mm

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

Bug: 927333
Change-Id: I55fa8ad3a62c89d99cfcf91d9b37427f7dace6c9
Reviewed-on: https://chromium-review.googlesource.com/c/1448090Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#628496}

ios/web_view/internal/signin/ios_web_view_signin_client.mm

@@ -5,7 +5,6 @@
 #include "ios/web_view/internal/signin/ios_web_view_signin_client.h"
 
 #include "components/signin/core/browser/cookie_settings_util.h"
-#include "components/signin/core/browser/device_id_helper.h"
 #include "google_apis/gaia/gaia_auth_fetcher.h"
 #import "ios/web_view/internal/sync/cwv_sync_controller_internal.h"
 #include "services/network/public/cpp/shared_url_loader_factory.h"